Definition:
While the rest of the United States of America (and, indeed, probably the rest of the English-speaking world) waits "in line" (or "in a queue," for the Brits), New Yorkers, of course, are different. They wait "on line."
Usage:
I saw Jerry Seinfield while waiting on line for coffee at Starbucks this morning.
Explanation:
Because New Yorkers just have to be different! (If not grammatically correct.)
