The morning after pill is a pill that can be taken by a woman after she has had sex to avoid pregnancy. It contains a high dose of hormones, and can therefore cause the user to feel rather sick.
The morning after pill is not to be advised as one's main means of contraception because of its strong side effects. However, it is useful as a back-up when other means of contraception have failed - for example, if one has forgotten to take the pill or when a condom was torn during sex.
Not to be confused with RU-486, the so-called "abortion pill". From a medical standpoint the morning-after pill is used to prevent pregnancy in the first place, while RU-486 causes the fetus to abort in an already pregnant woman. However some religions find no moral distinction (as indeed they reject all mechanical forms of contraception.)