Opposite of online. Temporarily or permanently disconnected from a network such as the internet, a BBS or any kind of online service such as Compuserve or AOL.
Especially refers to inactive dial-up connections over phone lines, but also network outages.

From Usenet jargon to the office:
Not now or not here. "Let's take this point offline." Used in meetings for items that come up that need discussion but are not on the agenda, and do not need input from everyone at the meeting.

(Often confusing to me because I work for a dot.com and in meetings we are usually discussing online products. I hear the phrase and do a double take as I sort out the appropriate meaning of the word.)

off-by-one error = O = ogg

offline adv.

Not now or not here. "Let's take this discussion offline." Specifically used on Usenet to suggest that a discussion be moved off a public newsgroup to email.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.