A universal at each moment in time, from the point of view of an observer, is a set of particulars.
Sybil Wolfram[3] writes: Particulars include only individuals of a certain kind: as a first approximation individuals with a definite place in space and time, such as persons and material objects or events, or which must be identified through such individuals, like smiles or thoughts.
In formulating a solution to the problem of universals, the term 'particular' can be used to describe the particular instance of redness of a certain apple as opposed to the 'universal' 'redness' (being abstract).
The term particular is also used as a modern equivalent of the Aristotelian notion of individual substance.
Used in this sense, particular can mean any concrete (individual) entity, irrespective of whether it is spatial and temporal or not.