An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

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).

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.

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

1) Occurring, done, or completed in an instant.
2) Existing at or pertaining to a particular instant (Dictionary.com).

→ instant; → -an; → -eous.

The → acceleration of a particle at time t defined by a = lim Δv/Δt = dv/dt. It is the limiting value of Δv/Δt at time t as both Δv and Δt approach zero.

→ instantaneous; → acceleration.

The velocity of a particle at some one instant of time, or at some one point of its path. It can be defined as the limiting value of the average velocity when the second point is taken closer and closer to the first point.

→ instantaneous; → velocity.