The Oxford English Dictionary offers seven unique definitions of the word film. As a means for situating film in the broader context of media, and as a means for handling the range of ways that film can be understood as a medium, it will be fruitful to initiate this reference article by making explicit two central definitions of the word.
First, film is the material basis for media such as photography and motion pictures. The Oxford English Dictionary defines film in this capacity as "a thin, flexible, transparent material consisting essentially of a plastic base or support (formerly of celluloid, now commonly of cellulose acetate) coated on one side with one or more layers of emulsion and sold as a rolled strip and as separate sheets; also, a single roll of this material, allowing a small number of exposures for use in still photography or a large number for use in cinematography."  Second, film is often used synonymously with words such as "movie" or "motion picture." The Oxford English Dictionary defines film in this capacity as "a cinematographic representation of a story, drama, episode, event, etc." 
Indeed, defining what a film is has proven to be one of the central tropes in film discourse. French film critic Andre Bazin published a four-volume tome whose title, What is Cinema?, defines its subject. It is from the first volume that his seminal essay "The Ontology of the Photographic Image" derives. In the essay Bazin argues that to understand what cinema is one must attend not only to the ontology of the actual film but also the psychological conditions governing its reception. For Bazin the invention of photography constitutes the most important even in the history of film. It was a moment when "a shift in the material, ontological basis of images forced a reconfiguration of aesthetics and psychology in the ever-variable 'balance between the symbolic and realism.'"  Realism, for Bazin, is "an automatic effect of photographic technology drawing on an irrational psychological desire"  and so it is psychologically, rather than aesthetically, that photography, and therefore cinema, satisfies "our appetite for illusion by a mechanical reproduction in the making of which man is no part." 
Noel Carroll, in the Encyclopedia of Aesthetics, attempts to make a push towards determining how one is to differentiate the film image from pro-filmic reality, painting, still pictures, and theater. His preliminary definition is constructed by situating film in opposition to other media in an attempt to isolate what is unique to film. He asserts that a film image is a "detached display," meaning that "seeing a film image is not just like seeing reality."  He notes that that while paintings are also detached displays, films differ in that they offer the possibility for "the impression of movement."  A stage play, however, is both a detached display and offers the possibility for the impression of movement, and so he revises his necessary conditions so that "X is a motion picture image (1) only if X is a detached display, (2) only if X is a member of the class of things wherein the impression of movement is possible, and (3) only if a token performance of X is generated by means of a template that is itself a token." 8 The template of such a performance that is itself a token of the performance is the physical filmstrip, which exists on its own after the token performance is over.
This final definition centers not only on film's material existence, but also the ways in which film is unlike other, potentially overlapping, media. However, other film theorists, including Carroll, have argued that conceiving of film as a unique, singular medium is wrong. For Carroll the notion of an artistic medium is vague, "referring sometimes to the physical materials out of which artworks are constructed, sometimes to the implements that do the constructing, and sometimes to formal elements of design that are available to artists in a given practice."  With regard to film, Carroll notes that photochemical emulsion is not necessary for film—some films are of projected images that have been painted onto a filmstrip—and neither is a camera. "Cinema can be made without cameras, a point reinforced by the existence of scratch films."  Carroll argues that "each artform is a multiplicity of media, and that the relevant media are open to physical re-invention."  Film, therefore, should not be understood as a unique, or even single, medium, but as many media.
Whereas Carroll, in writing about multiplicity of media contained within film, is concerned particularly with the content of film, other scholars have formulated theories of film that encompass elements beyond the film proper. Jean-Louis Baudry developed the idea of film as an apparatus, where film is understood not only by content of the film but also with respect to the camera, the film stock, the editing, the projection, etc. Namely, all the elements of film technique and technology that go into producing a film. In “Ideological Effects of the Basic Cinematographic Apparatus” Baudry argues that “the specific function fulfilled by the cinema [is] as support and instrument of ideology.” 12 Crucial to his understanding of film as an ideological instrument is the spectator. “The ideological mechanism at work in the cinema seems thus to be concentrated in the relationship between the camera and the subject.”  Cinema, then, is a “sort of psychic apparatus of substitution, corresponding to the model defined by the dominant ideology” that constitutes its subject to serve the dominant ideology. 
A key component to Baudry’s argument about the ideological effects of the cinematographic apparatus is that film projection enables the illusion of continuous action even though film is composed of discrete images which, in most films, is a string of minutely different photographic images. It is easy to think of photography as the antecedent to film; that the relationship between the two is natural or evolutionary. Indeed, the discovery, in the 16th century, that exposed salts of silver would darken if exposed to light enabled the knowledge and technology necessary for the first daguerreotypes in the early 1800s. These daguerreotypes eventually gave way to the first modern photographs in the middle of the 19th century, culminating in 1887 when Eastman introduced celluloid as the material basis for photography. Celluloid-based photography would go on to serve as the material basis for most modern films.
But the history of film is not a clean, linear narrative. The years of early cinema are marked by disparate, competing technologies, all of which can lay claim to heralding in the modern film and each of which has informed and shaped what eventually became standardized, in the early 20th century, as the modern cinema. Though most technologies were photography-based, the Mutoscope (19th century) and Zoetrope (19th century), for example, were devices that functioned in ways principally similar to film projection. Instead of a filmstrip moving in front of a flickering light, these devices quickly rotated images in front of peepholes to create the illusion of moving images. And just as no single technology is the immediate precursor to film, the relations hip between film and photography is not simply evolutionary—the advent of cinema did not spell the demise of photography. As much as the technologies overlap, there are crucial differences between the film and photography including, for example, the possibility of synchronous sound in film.
As Paolo Cherchi Usai writes, "The history of cinema did not begin with a 'big bang.' No single event—whether Edison's patented invention of the Kinetoscope in 1891 or the Lumiere brothers' first projection of films to a paying audience in 1895—can be held to separate a nebulous pre-cinema from cinema proper. Rather there is a continuum which begins with early experiments and devices aimed at presenting images in sequence and includes not only the emergence in the 1890s of an apparatus recognizable as cinema but also the forerunners of electronic image-making." 
Just as the origins of film are marked by the existence of competing image technologies so to is in the current film world. The rise of digital imaging technologies over the last few decades is challenging film as the material basis for cinema. Increasingly films are being edited with non-linear editing programs, which require the analog film stock to be digitized so that the film can be edited on computers. Computer Generated Images (CGI) have replaced older, analog special effects techniques, some of which, such as superimposition, date back to the early twentieth century and are almost as old as cinema itself. Lee Manovich has argued that "conceptually, photorealistic computer graphics had already appeared with Felix Nadar's photographs in the 1840s and certainly with the first films of Georges Melies in the 1890s. Conceptually, they are the inventors of 3-D photorealistic computer graphics."  Then there is digital video, a medium on which feature-length films are slowly being recorded. While analog film is still the basis for most major studio movies, the digital has invaded cinema to the point that, for Manovich, "[cinema] is no longer an indexical media technology but, rather, a subgenre of painting...The photographic and cinematic aspects of cinema are being replaced by the painterly and the graphic."  The influence of digital media is altering, perhaps fundamentally, what it means to be a film. Or, as Manovich puts it: "The moving-image culture is being redefined once again."