Definitions for "First Day Cover"
Cover franked with a postage stamp or set of stamps postmarked on the first day of issue.
cover cancelled on the day a stamp is issued, in the city in which it is released. (Modern First Day Covers may actually be cancelled up to 30 days after the date of issue, and not necessarily in the city whose postmarks they bear.) First Day Covers may be addressed or unaddressed, and with or without cachets. See Unofficial First Day Cover and Second Day Cover.
A cover bearing a stamp (or set of stamps) posted on the day of issue. It is a very popular collectible among philately buffs. First Day Covers usually have an illustration depicting an aspect of a subject of the issue and often include a card or info sheet which gives more details about the subject.