When you are busy, you are working hard or concentrating on a task, so that you are not free to do anything else.

A busy time is a period of time during which you have a lot of things to do.

If you say that someone is busy thinking or worrying about something, you mean that it is taking all their attention, often to such an extent that they are unable to think about anything else.

If you busy yourself with something, you occupy yourself by dealing with it.

A busy place is full of people who are doing things or moving about.

When a telephone line is busy, you cannot make your call because the line is already being used by someone else.