adjective
Of a system, currently not connected (generally electrically) to a larger network. For example, a power plant which is not connected to the grid, or a computer which is not connected to the Internet or to any other communications service.
(by extension) Happening in the physical world (the real world) as opposed to on the internet.
adverb
While offline; with an offline system.
(business slang) Outside the current meeting, in a more private setting.
verb
(computing, transitive) To take (a system, etc.) offline; to demote from an active or online state.
(video games, Rust) To raid another player's base while that player is not currently playing.