A. Atomism is the name of a theory or kind of theory. (The derivative adjective 'atomistic' can apply to views or things consonant with some form of atomism). In its most general sense the term refers to any doctrine which maintains that the subject-matter of a given discipline is divisible into a set of units which are not divisible further and which are thus its ultimate constituents. Alternatively, the term can also be applied to a doctrine which allows further divisibility of units, but insists that all larger units are merely aggregates of smaller ones. (For instance, a social theory is atomistic if it insists that all social institutions and events are to be seen as composed of the actions of individual men and nothing else, even if the doctrine allows that individual men may be seen as divisible by some other science). In general, atomistic doctrines tend to be inspired by the view that unless the process of division and subdivision comes to a rest when it reaches some kind of ultimate unit, there can be nothing real to investigate, for systems or aggregates do not 'really exist' over and above the 'parts'. In practice, it is not so much the theoretical defects of this argument, as the practical difficulties of locating and identifying plausible 'atoms' and showing that real structures and their behavior can be exhaustively explained in terms of them, which have led to doubts about atomism.
B. Atomism becomes the name of a specific doctrine rather than of a type of doctrine when the context in which it is to be applied is either tacitly understood or specified, usually by an adjective (e.g., physical, logical, social).
1. Thus, in sociology or political science, atomism may designate a theory to the effect that social processes and groups are by-products of the doings of social atoms. Individual people or their individual acts may be identified as these atoms.
2. In the psychology of perception or in epistemology, atomism will designate a theory to the effect that there are ultimate units of perception or sensation, and that percepts and perceptible objects generally are to be understood in terms of them.
3. In logic and in ontology, atomism is to be understood as the doctrine that there are ultimate units of discourse, or of existences, from which all meaningful assertion, or all being is built up.
Taken from A Dictionary of the Social Sciences eds. J. Gould and W. Kolb, Free Press, 1964.