The action of smelling; the capacity for smelling; the sense of smell.
Either derived from French olfaction, sense of smell (1825; 1530 in Middle French as olefaction; 1507 in sense ‘odour’) or drom classical Latin olfact-, past participial stem of olfacere, to smell. 
As with the other senses, olfaction can be described using the language of both science and art. However, unlike sight, hearing, taste, and touch, the sensations of olfaction often evade tidy linguistic description, a difficulty noted by Plato and Aristotle as being one of the main characteristics of the sense. Aristotle attributed its inherent uncertainty to the relative atrophy of the human faculty, which cannot sharply distinguish discrete identities but merely associates smells with corresponding emotions.  Plato also characterized olfaction by its vague, emotional sensations, but explained this imprecision by the fact that the pure elements do not smell, and thus smells are only half formed, relying on material in mixed states. 
Modern findings in neuroscience may suggest an anatomical explanation for the sub-verbal, emotional nature of smell. The other senses all have structures in the neo-cortex, the higher brain, but olfaction is processed by a system distributed over both the higher cortex and the limbic system, the ancient lower structures of the brain responsible for emotion, motivation, and childhood memories.  Smell can bypass the mediation of cortical relays , and this direct link to the sub-rational may give olfaction its characteristically indistinct yet powerful sensations.
While most Western thinkers decoupled olfaction and taste from the intellect, this segregation is not universal. For example, Kant considered the pleasures of olfaction "fleeting and transitory," unworthy of cultivation.  However, Nietzsche values the subrational, intuitive power of olfaction, claiming that he could perceive falsehoods by "smelling them out."  The Onegee of Little Andaman Island (near Bengal) use smell as the model for their cosmology and epistemiology, conceiving of smell as the life force and of space and time as fluid and varying with the season, as do the odors of their environment.  The Western exclusion of olfaction from the higher, intellectual senses may stem from its resistance to concrete classification of its sensations. A number of cognitive studies found that odors are perceived on a non-hierarchical continuum, and identification of isolated odors is inherently subjective. With olfaction, one distinguishes qualities, not identities.  Precise linguistic representation of scents is therefore difficult, as can be described using Peirce's theory of signs.
A word is a symbol of scent-experience through scent-words is inherently indistinct. For example, although perfumers have compiled lists containing thousands of pure odor substances, the descriptors of such scents list only in the hundreds.  Even for these professional noses, equipped with specialized vocabulary, it is clear that the lexicon of scent is far surpassed by the diversity of odors, without even considering the nearly limitless number of possible combinations. It is no surprise then, that relation of a olfactory sensation through language is inherently imprecise.
When scientists seek to precisely represent scents, they use complex shape metaphors. By inserting electrodes directly into the brains of fruit flies, researchers were able to determine the neurological response of the flies to a host of scents.  These raw neurological data are the exact object, the second, of the chemicals presented to the flies. Although these flies lack descriptive language, and thus a first (representamen) and third (interpretant), there is no reason to believe that olfaction is fundamentally different in humans. The representation of this scent-second is mapped onto a three dimensional space comprised of the responses of three different types of olfactory sensors. Mango extract, for instance, results in a constellation of electrical responses that, if united by surface, would form a shape unique for that smell. As the potency of the extract is decreased, the shape contracts, which means that the different elements of the scent are less and less distinct. Only a sculptural metaphor encodes enough information to be a precise representation of a scent. However, this visual abstraction conveys none of the emotional associations an actual scent can.
Despite the number and complexity of scents, we can immediately classify a scent as pleasant or repulsive. This intuitive discernment seems to be based in learned templates for scents. Children must be taught to abhor excremental smells, which are universally detested.  This implies that even the most fundamental disgust towards odors must be learned. Several experiments found that the perception of scent is dependent on experience and memory, indicating that olfaction is a learned skill. Subjects were presented with six distinct odors divided into pairs, AX and single odors, B and Y, and then were asked to group the odors by similarity in a blind test. A and X were perceived to be significantly more similar, despite the actual identity of the chemicals, and were difficult to distinguish separately (for example, A versus AX).  This indicates that smells are learned by association, and not by a distinct sensor for every possible scent. The ability to distinguish scents can be greatly increased by experience, as seen in expert perfumers, and an exceptional case of an amnesiac who could distinguish the intensity but not type of smell, that is, everything smelled the same to him.  These findings strongly indicate that memory and experience plays a role in olfaction. Interestingly, memories of odors retain more poignancy than the those of other senses.  However, since these memories are closely linked to the experience in which the scent was learned, they can evoke emotions from this time period, a powerful experience that has been anecdotally reported by many. This emotional association is lost in trained experts, who can perceive and name many more odors, but have learned and re-learned these sensations in a sterile, professional environment. 
If olfaction is the result of a direct link between chemical and the induced scent, then there must be a learned template, an artificial icon in the mind that allows one to categorize a new scent. This process is analogous to the formation of an image, an objet petite a, on the screen of Lacan's theory of the gaze.  The scent-image is learned, and it is this image that is perceived instead of the real subject of representation, the chemical substance. In the fruit-fly model, a new scent creates a slightly different shape in olfactory space (the responses of the different sensors in ones nose), and this unique shape is assigned a scent-image to represent the particular mix of chemicals that is perceived.
In analogy with conventional images, these scent-images allow one to distinguish otherwise undetectable scents, but also have similar complications. As a representation, the scent-image does not convey the full reality, and thus when applied to human interaction it can act as a veil preventing face-to-face encounter. Just as stereotypes encompass the image of the other, so can they encompass the scent. W.J.T Mitchell observes that " ...the status of these pictures is so slippery and mobile, ranging from phenomological universals, cognitive templates, to virulently prejudicial distortions..."  Scent-images likewise are necessary for perception, but can also seductively obstruct reality. A historical example of this is the "Foetor Judaicus," a stench that Medieval Europeans claimed emanated from the Jews.  In modern society, these olfactory stereotyping can be seen in ethnic food-smell prejudices, in which the characteristic ingredients of a group's diet, curry or fish-paste for example, are impugned as foul-smelling and treated as pollution and a mark of low-class. Also, scent is an immediate indicator of hygiene, acting as an immediate indicator of extreme poverty. In Spike Lee's Bamboozled, the appearances of the street-dwelling Man-Ray and Womack arose suspicion, but the smell of their unwashed bodies repulses the security guards and confirms their outcast status. Thus as representations, scent-images share the complexity of their visual counterparts, acting as both enablers of perception and veils to communication.
Of the senses, Olfaction joins us the most intimately with our environment. Taste has an element of choice, and the sources of the other sensations are remote. The sensation of olfaction is caused by chemicals entering the nostrils, penetrating the body without consent.  As such, olfaction has distinct properties as a medium for communication. Warning of a gas leak is communicated through artificial putrescine, the stench of rotting flesh. The state of one's hygiene is immediately communicated through olfaction, and the vast soap, deodorant, and perfume industries capitalize on the crucial importance of smelling pleasant. However, only recently has another avenue of communication been proven in humans.
Pheromones are chemical signals that deeply influence factors such as sexual attraction and synchronization of ovulation, yet are undetectable by conscious perception. Researchers at the University of Chicago have proven that pheromones coordinate ovulation in groups of women living in close quarters.  Also, smell has been implicated in womens' selection of men with a different set of immunity genes (increased diversity of antibodies mean healthier offspring).  Pheromones directly manipulate the biology of the interpretant, bypassing conscious perception. In this way, they are the starkest example of the power of olfaction - bypassing the rational, higher structures of the brain, scents directly trigger emotional responses.