Vagina

The vagina (from the Latin for "scabbard") is the tubular tract leading from the uterus to the exterior of the body in female mammals, or to the cloaca in female birds and some reptiles; insects and other invertebrates also have a vagina, which is the terminal part of the oviduct.

For the purposes of anatomy, a vagina can also be any structure that serves as a sheath (or theca), as in, the vagina of the portal vein. Another example is the fibrous sheath around tendons, called a vagina fibrosa when solid or a vagina mucosa when it contains a fluid-filled cavity around the tendon.

non-Human Mammals

In mammals, the exterior vaginal opening is the vulva. A membrane situated behind the urethral opening, the hymen, partially occludes the vagina in many organisms, including some human females, from birth until it is ruptured by first coitus, or by any number of other activites including medical examinations, injury, certain types of exercise, introduction of a foreign object, et cetera.

The vagina serves the purposes of:

providing a path for menstrual fluids to leave the body,
admitting the penis of the male for coitus and ultimately the introduction of male gametes (sperm) for the fertilization of ova,
providing a route to deliver a fully gestated fetus from the uterus to its independent life outside the body of the mother in the process of live birth. During birth, the vagina is referred to as the birth canal.

Humans

Female internal sexual anatomy

A woman's external genitals are often referred to as "vagina", but strictly speaking, this is the vulva or pudenda (Latin shame), as distinguished from the interior vaginal tract.