You ostensively define something when you point to something and name it.  For example, if someone were to say, "What's a baboon?" you might point to one (or better still to a picture of one), and say "That's a baboon."  

Wittgenstein organizes much of his thinking around his critique of the idea that ostensive definitions are the major way we learn language. Wittgenstein's first section of his book, The Philosophical Investigations, shows us why this is an insufficient answer, and in place of the ostensive definition, blossoms a whole new picture of how language works.

Want to see the first part of the Philosophical Investigations?  If so, click here.