An Etymological Dictionary of Astronomy and Astrophysics

1. Occurring or accomplished without delay; following or preceding without a lapse of time. → instant.
   

2. Having no object or space → intervening; without intervening medium or agent; direct (Dictionary.com).

Etymology (EN): M.E. immediat, from O.Fr. immediat, from L.L. immediatus “without anything between,” from → in- “not, opposite of,” + mediatus, p.p. of mediare “to be in the middle,” from L. medius “middle,” → medium.

Etymology (PE): Âni, from ân loan from Ar. ân “time, a short time.”
Amadim, literally “with nothing interposed,” from the negation prefix → a- + madium, → medium.