means The moment of suddenly understanding or becoming fully aware of something — or, separately, the act of making something real or achieving it.

from From the verb "realize," which came into English in the early 1600s from the French "réaliser," built on "réel" (real) — itself tracing back through Latin "realis" to "res," meaning "thing." So at its root, to realize is to make something a "thing": either by bringing it into existence or by letting it land, at last, as a thing in your mind. The "-ation" ending simply turns the act into a noun — the having-realized.