If you defy someone or something that is trying to make you behave in a particular way, you refuse to obey them and behave in that way.

If you defy someone to do something, you challenge them to do it when you think that they will be unable to do it or too frightened to do it.

If something defies description or understanding, it is so strange, extreme, or surprising that it is almost impossible to understand or explain.

If you say that someone defies their age, or defies the years, you mean that their appearance or behaviour suggests that they are younger than they really are.