Most people didn't understand what Bill Clinton meant when he said »that depends on what the definition of 'is' is.« The question placed was »Is there a sexual relationship...«
Clinton had to be careful. If he answered »no,« they could get him on the fact that there had been a relationship in the past, even though there may not now »be« a relationship.
If »is« means »currently exists« the answer might have been no, but if you remove the temporal element it could possibly have been yes.
When you're being questioned under oath, you have to be sure of your definitions, otherwise you may later be accused of lying or you might admit something you had no intention of admitting.