A "jazz standard" is a composition that is held in continuing esteem and is commonly used as the basis of jazz arrangements and improvisations.A simple, succinct and satisfying defintion, I'd say. Here's a terrific exploration of the rich and delightful world of Jazz Standards. I discovered this wonderful site via this page.
(And for the Lazy Jazz Geek in you, this is the list of the Standards.)
P.S.: Any other music geeks miss the good old All Music Guide?