An Etymological Dictionary of Astronomy and Astrophysics

1a) An → infinitesimal or very short space of time;  a → moment.

1b) The point of time now present or present with reference to some action or even.

2a) Succeeding without any interval of time; prompt;  → immediate.

2b) Occurring, done, or prepared with a minimal amount of time and effort; produced  rapidly and with little preparation (Dictionary.com).

Etymology (EN): M.E., from O.Fr. instant (adj.) “assiduous, at hand,” from M.L. instans-, in classical L. “present, pressing, urgent,” literally “standing near,” pr.p. of instare “to urge, to stand near, be present,” → insist.

Etymology (PE): 1) Lahzé, from Ar. laHZat, laHZa “glance; moment.”
2) Setand (variant of istandé), from setâdan, variant of istâdan “to stand,” → instance.