Meaning "condense" is from 1680s; that of "intensify the action of" is from 1758. Sense of "mentally focus" is from 1860s, on the notion of "concentrate the mind or mental powers." Related: Concentrated; concentrating.
"that which has been reduced to a state of purity," 1883, from concentrate(adj.) "reduced to a pure or intense state" (1640s), from concentrate (v.).