The sac that contains the amniotic fluid (or amnii) and the developing embryo/fetus in pregnant mammals, reptiles, and birds.
The sac is composed of two layers. The thin tough inner membrane of the amniotic sac is called the amnion and is primarily protective in function. The outer layer is called the chorion. The chorion plays an important role in carrying nutrients to the developing embryo.
The sac is also known as the bag of waters; when the amniotic sac ruptures (generally as labor begins), it is called water breaking (as in, "my water broke"). This is accompanied by the loss of amniotic fluid from the vagina, and is normally a sign that the baby will be coming out shortly.
Giving birth with the amniotic sac partially or wholly intact is called a veiled birth, or, often, being 'born with the caul'. This has a number of superstitions surrounding it, including that the baby will be gifted with some sort of mystical powers or that the baby is destined to be a midwife. Babies don't need to take a breath for a few minutes after birth (which is why water births work), so there is generally no danger in veiled births.