aggregate is a whole formed by collecting separate units into one mass or sum — and the same word names the act of collecting them and the quality of having been collected.
The word carries its meaning across three grammatical roles without changing it. As a noun it is the total: "in the aggregate" marks a claim as holding across a population rather than case by case. As a verb it is the gathering, transitively bringing parts into a mass and intransitively amounting to one. As an adjective it describes what such gathering produced. Only the pronunciation shifts, on the pattern of separate and moderate, where the verb keeps a full final vowel that the noun and adjective reduce.
Several fields have claimed it as a term of art, and each keeps the same core. In construction, the aggregate is the sand, gravel, and crushed stone that cement binds into concrete. In two-legged sporting ties, the aggregate score is the combined result of both matches. In economics and statistics, aggregate demand and aggregate data name totals computed over a population instead of individual observations. What unites the uses is that the constituents are still there — counted as one, not dissolved into one.
The verb reached English around 1400 from the Latin aggregatus, past participle of aggregare, "to add to a flock, to gather," from ad-, "to," and gregare, "to collect into a flock," from grex (gregis), "flock" — the root behind gregarious, congregate, and segregate. The buried metaphor is herding, and it explains the word's persistent implication that the parts remain countable. That is the line between aggregating and other kinds of joining: to 📝coalesce is to grow together until the seams are gone, while an aggregate keeps its seams. It is likewise not 📝totality, which names wholeness as a state rather than a sum arrived at by addition.