The noun instant has 2 senses (first 2 from tagged texts)
1. (22) blink of an eye, flash, heartbeat, instant, jiffy, split second, trice, twinkling, wink, New York minute -- (a very short time (as the time it takes the eye blink or the heart to beat); "if I had the chance I'd do it in a flash")
2. (9) moment, minute, second, instant -- (a particular point in time; "the moment he arrived the party began")
Overview of adj instant
The adj instant has 3 senses (first 1 from tagged texts)
1. (2) instantaneous, instant -- (occurring with no delay; "relief was instantaneous"; "instant gratification")
2. instant, inst -- (in or of the present month; "your letter of the 10th inst")
3. clamant, crying, exigent, insistent, instant -- (demanding attention; "clamant needs"; "a crying need"; "regarded literary questions as exigent and momentous"- H.L.Mencken; "insistent hunger"; "an instant need")