Definition:
A #10 envelope is a standard business-sized envelope which measures 4 1/8 x 9 1/2 inches. This term is used in sweepstakes rules to indicate the type of envelope that you have to use to send in your entries. Be sure to stick to the rules carefully, because using a letter-sized envelope could disqualify you from the sweepstakes and destroy your chance of winning.

Also Known As:
standard business envelope, business-sized envelope,

Alternate Spellings: No. 10 envelope, Number 10 envelope

Examples:
Mail your original entry form to us in a #10 envelope.