If something happens twice, there are two actions or events of the same kind.

You use twice in expressions such as twice a day and twice a week to indicate that two events or actions of the same kind happen in each day or week.

If one thing is, for example, twice as big or old as another, the first thing is two times as big or old as the second. People sometimes say that one thing is twice as good or hard as another when they want to emphasize that the first thing is much better or harder than the second.

If you think twice about doing something, you consider it again and decide not to do it, or decide to do it differently.