morning-after pill

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various oral drugs that are intended to prevent pregnancy for up to five days after sexual intercourse by delaying or inhibiting ovulation, preventing fertilization of an egg, or preventing implantation of a fertilized egg.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a contraceptive in the form of a pill taken the morning after sexual intercourse

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a large dose of estrogen taken orally within 24 to 72 hours after intercourse; prevents implantation of a fertilized ovum and so acts as a contraceptive; commonly used after rape or incest