When connected to another computer via modem and telephone lines, a modem is said to be online. When disconnected, it is offline.
When someone is ‘online', he/she is connected to the Internet. People communicate online by sending and receiving information via email, instant messaging or a chat room. Offline is another term for the real world (i.e. outside the Internet).
You are online when you are connected to the Internet via your Internet service provider. You can choose to save pages to be viewed offline when you are not connected to the Internet. You can also read and compose responses to downloaded e-mail while you are offline. Then, when you sign in again, you can send the e-mail you wrote while offline.
When connected to another computer via modems and telephone lines a modem is said to be on-line. When disconnected, it is off-line. (7/96)
When your computer is connected to the Internet it is 'online'. When the Internet connection is turned off, you are 'offline'.
When you are logged onto a computer through your modem, you are said to be online. When you are using your computer but are not connected to a computer through your modem, you're said to be working offline.