What does "instant" mean?

Additional senses

1. 2.Closely pressing or impending in respect to time; not deferred; immediate; without delay. Impending death is thine, and instant doom. Prior.
2. 3.Present; current. The instant time is always the fittest time. Fuller. Note: The word in this sense is now used only in dates, to indicate the current month; as, the tenth of July instant.
3. 4.Instantly. [Poetic] Instant he flew with hospitable haste. Pope.
4. 5.A point in duration; a moment; a portion of time too short to be estimated; also, any particular moment. There is scarce an instant between their flourishing and their not being. Hooker.
5. 6.A day of the present or current month; as, the sixth instant; -- an elliptical expression equivalent to the sixth of the month instant, i. e., the current month. See Instant, a., 3.