I don't know if there is a universal definition. Here in the U.S. they usually use the term Prawn when it's the large shrimp. In the nicer restaurants here the most popular is the Santa Barbara Spot Prawn, delicious!
At Romy's, 1 lb. of Prawns = 3-5 pieces of crustacean. The regular shrimp you get about 15-20 in 1 lb.