tense (noun): a form of a verb used to indicate the time, and sometimes the continuation or completeness, of an action in relation to the time of speaking. (From Latin tempus = time).
Tense is a method that we use in English to refer to time - past, present and future. Many languages use tenses to talk about time. Other languages have no tenses, but of course they can still talk about time, using different methods.
So, we talk about time in English with tenses. But, and this is a very big but:
- we can also talk about time without using tenses (for example, going to is a special construction to talk about the future, it is not a tense)
- one tense does not always talk about one time (see Tense & Time for more about this)