We have put together a list of words that are similar to INSTANTANEOUS.

2 Alternative Words Similar to instantaneous

1 Instant Adjective      Synonym Words Like Instant
2 Fast Adjective      Similar Words Like Fast

3 examples of instantaneous

1 the passage of electricity appears to be instantaneous
2 instantaneous acceleration, velocity, etc.
3 instant gratification

9 definitions of instantaneous

1 Done or occurring in an instant, or without any perceptible duration of time.
2 At or during a given instant
3 occurring with no delay
4 Occurring or completed without perceptible delay: Relief was instantaneous.
5 Done or made as quickly or directly as possible: an instantaneous reply to my letter.
6 Present or occurring at a specific instant: instantaneous velocity; instantaneous pressure.
7 Done or produced in an instant; occurring or acting without any perceptible lapse of time.
8 In mech., existing in or referring to an instant of time; momentary: as, instantaneous position, displacement, velocity, acceleration, etc. (that is, the position, etc., at any instant).
9 Occurring, arising, or functioning without any delay; happening within an imperceptibly brief period of time.
We get our data from many different dictionaries across the web:
Wordnik, Wiktionary, Century, American Heritage, Gcide