Intension sounds like "intention," and it's just one letter off, but it means something else. It's a term used in logic and linguistics to refer to the concept that a word evokes.
Intension is a common misspelling of intention, but the two words shouldn't be confused. If you're talking about an aim or purpose, then you want intention. Intension with an "s" should only be used in technical discussions about the meaning of language, or semantics. The intension of the word "dog" is the whole concept of dog-ness that the word brings to mind.