On the definition of memory, the Oxford English Dictionary (OED) outlines the various functions of this complex and varied noun. It offers the cognitive function of remembering, the physical site of retention and custody of sensory experiences of the past. The theme of corporeality continues in a discussion of memory as the capability of an organism to manifest the previous effects or state in another setting, to retain an impression from a past experience. Similarly, the OED discusses memory as subject-specific, by which I mean outlined as a personal repository of experiences. It posits memory as a middle ground for the unconscious, the place that acts as a medium of recovery from the inaccessibility of the unconscious. Implicit in this is the accessibility of memory, the interaction of the will on the brain to recover, to preserve sensory experiences.
Singular instances, impressions, also are signified by the term memory. The OED offers such moments as recollections, acts or instances of remembering, specific persons or things remembered, or the fact or condition of being remembered. Similarly, the word can connote a loss or an absence, as in the sense of a memorial or for a person or state no longer present. Objects can also function as such, in the sense of a physical, symbolic replacement for something lost or gone: a memento, monument, or memorial would be an example.
Implicit in the above defined uses are that memories are simply an impression of a past experience, importantly that they are no longer present, but rather are a retrieval of a particular previous moment. In this manner, memory functions as a medium of storage, as an intervening substance between temporally past sensory impressions and present consciousness. Memories are a temporal channel, a time machine of sensory traces of the past.
The Encyclopedia of Philosophy offers that memory need not refer exclusively to the past, that one could remember an event that is presently occurring or will occur sometime in the future, but argues that memory most often refers to a past experience. "Despite this variety of uses, philosophers writing on memory have tended, until recently, to concentrate on those uses of "remember" in which it takes as its object an expression referring to a particular past event or action."  The OED also points to the importance of temporality for the concept of memory, showing temporality as an element of recollection, specifically the span of time in which a reminiscence passes. Movement along the temporal echoes the process of sensory experience, and links the issues of memory to the Hegelian problematization of sense-certainty.
Frances Yates chronicles the historical usages of memory in rhetoric in her article Three Latin Sources for the Classical Art of Memory. Utilizing a spatial conceptualization of mnemonic processes, Roman rhetors were capable of recounting lengthy orations with little difficulty or error. By a process of visualization of the space of memory, one could 'place' elements in a linear movement throughout the conceptualized architecture and recapture them through 'moving' again through this image. Yates also likens the mnemonic process to linguistic structures, stating "(t)he art of memory is like an inner writing"  . By imagining a spatial inscription or attribution, the orator would be able to revisit the symbolic recollection and summon up the information.
Similarly, Walter Benjamin reflects on the social uses of memory in his essay entitled The Storyteller. He contrasts storytelling, a communicative form relying solely on memory, to information, which he defines as the communications of modernity. He sees memory functioning as a medium between generations and varied experiences. " Memory creates the chain of tradition which passes a happening on from generation to generation...in the first place among these is the one practiced by the storyteller. It starts the web which all stories together form in the end.  Memory, and more specifically, collective memory, serves to unite and make links between the generations, provide a sense of shared heritage.
Shared remembrances, as distinct from individual memories, sit often in a contested place. I would define Collective memory as the communal narratives regarding a past event that enjoy relative acceptance or consensus by a group. There remains however, the possibility of conflict between professional historical methodology and collective knowledges. In the telling of previously marginalized or ignored histories, conflicting narratives raise the question of validity in memory. The accuracy of memory as a media of information regarding the past becomes politicized in this discrepancies. Culture wars occur increasingly around issues of representation and the past. Public History organizations and exhibits present an arena for such disputes. One need only look to the debate regarding the Smithsonian's National Air and Space Museum's 1994 exhibit on the WWII plane the Enola Gay for an example of contestations over the past. The controversy surrounding the proposed exhibit and ensuing backlash from veterans claiming historical revisionism in the name of political correctness cogently portrays the potential contestation between professional history and collective memory, as well as between different collectives.
In his chapter "Narrative, Memory, and Slavery," W.J.T. Mitchell problematizes the notion of memory as a direct representation of the past. He argues "representation... not only 'mediates' our knowledge...but obstructs, fragments, and negates that knowledge...(memory) provide(s) something more like a site of cultural labor, a body of textual formations that has to be worked through interminably"  . For Mitchell, memory is not interesting for what it tells us, but rather what it hides from us. Calling memory a "medium", he posits it as a process of meaning creation that is both selective and akin to a facade. In describing memory as "a technology for gaining freedom of movement in and mastery over the subjective temporality of consciousness and the objective temporality of discursive performance"  , Mitchell politicizes the function of memory. Rather than a recalling of a sensory input of the past, memory is a process by which a subject narrates the past, explains the experiences and gains power over the world he inhabits.
We can see how all the uses of memory as outlined above suggest a system of storage and a medium of recovery. Whether referencing the human mental capacity for storing past sensory traces or an artificial system or technology of retrieval, a temporal transmission and mediation is a necessary component of memory. The possible tension in discrepancies between academic history and collective memories demonstrate the politics of remembering and forgetting.