What Does Offline Mean?
"Offline" refers to a device not connected to a network. It may also be used for a device that is not working properly, such as an "offline printer." Offline can additionally refer to the "real world" outside of the internet. This is normally used to distinguish between online and offline communications, such as email vs. "snail mail."
Techopedia Explains Offline
At its most literal, "offline" means not connected to a network. Many people refer to the "internet" when they use the terms online and offline.
Others might refer to a device as offline when it is not working properly, whether it is networked or not. For example, "The printer is currently offline."
Many internet programs have an "offline mode" when disconnected from a network. A browser can still show pages that are either already loaded or local pages. Dedicated email clients let users view messages they have already downloaded when not connected to a network. They can also reply to them or compose new messages. When a user reconnects to the network, the messages are sent.
Many mobile devices also allow users to switch them into "airplane mode" which disconnects the Wi-Fi and cellular networks to comply with regulations on commercial flights.
The term "offline" is also used to refer to the real world, or "IRL" (in real life). A user in an instant messaging conversation might want to take the discussion offline. In other words, the user would rather have a face-to-face conversation.