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.
adjective: (by extension) Happening in the physical world (the real world) as opposed to on the internet.
adverb: While offline; with an offline system.
adverb: (business slang) Outside the current meeting, in a more private setting.
adverb: From ("off") the Internet
verb: (computing, transitive) To take (a system, etc.) offline; to demote from an active or online state.