Occurring or completed without perceptible delay: Relief was instantaneous.
Done or made as quickly or directly as possible: an instantaneous reply to my letter.
Present or occurring at a specific instant: instantaneous velocity; instantaneous pressure.
[From Medieval Latin īnstantāneus, from Latin īnstāns, īnstant-, present; see instant.] in·stan'ta·ne'i·ty (ĭn-stān'tə-nē'ĭ-tē, ĭn'stən-) n., in'stan·ta'ne·ous·ly adv., in'stan·ta'ne·ous·ness n.