An Etymological Dictionary of Astronomy and Astrophysics

The sequence of all natural processes in which the → entropy increases. In other words, the fact that these processes all move in one direction in time and are → irreversible. The past is distinctly different from the future; things always grow older, never younger.

Etymology (EN): → time; arrow, M.E. arewe, arwe, from O.E. arwan, earh “arrow,” from P.Gmc. *arkhwo (cf. Goth. arhwanza), from PIE base *arku- “bow and/or arrow,” source of Latin arcus, → arc.

Etymology (PE): Peykân “arrow,” → Sagitta; zamân, → time.