The view that only particulars exist, and more specifically that the properties and relations of particulars are themselves particulars, not universals (see Platonism).
A particular has a certain unity in space and time. It cannot appear as a whole at separated places simultaneously (though its parts may be scattered, as when an object is dismantled for repair), and normally (there are exceptions) can appear as a whole at different times only if those times form a period, without gaps.
Particulars need not be solid, and their parts may constantly change (shadows and flames are particulars, and so are events and actions). They are primarily contrasted with universals, which can be instantiated; no particular can be instantiated.
Particularism is similar to nominalism, but a particularist might accept abstract particulars like numbers, seen - as a Platonist would see them - as abstract objects; they are particulars in so far as they vacuously satisfy (that is, do not positively break) the requirements about being unified in space and time: one can have instances of twoness or duality, which is a universal, but not instances of two. A nominalist, however, would normally reject numbers seen as abstract objects.
D M Armstrong, Universals and Scientific Realism, 1, Nominalism and Realism (1978)