An
indefinite period of time, usually assumed by the listener to be at least 2-3 minutes in length. Speakers use it to
affirm that the time will be short without any actual meaning or
elaboration, despite the fact that the period will likely be more than 120 times longer than stated (see above).
Person #1: Are you done yet?
Person #2: Yeah, it'll just take a second.
-5 minutes later-
Person #1: *visibly
impatient* Okay, are you done now?
Person #2: I said, it'll be a second!
Geez, some people.