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1)A common phrase in the advertising world - which gives rise to interesting loopholes - which tends to mean that you need not make a purchase to be counted as a contestant in an event.

2)Also, generally is a lie and means if you don't purchase but wanter to enter a contest, they'll send you a form to make you "feel" like a contestant.
The Sprite "under the cap" game said "no purchase necessary" right on the bottle, so Justin and David opened up all the bottles in the store and kept only the winners.
by Justin 47 Nishi November 04, 2004