The vagina (from Latin vagina, literally "sheath" or "scabbard") is a fibromuscular tubular tract leading from the uterus to the exterior of the body in female placental mammals and marsupials, or to the cloaca in female birds, monotremes, and some reptiles. Female insects and other invertebrates also have a vagina, which is the terminal part of the oviduct. The Latinate plural "vaginae" is rarely used in English.

The word vagina is quite often used colloquially to refer to the vulva or female genitals generally; technically speaking, the vagina is a specific internal structure. In humans, the passage leads from the opening of the vulva to the womb and uterus. It lies midway between the anal tract and the urethra.

The human vagina is an elastic muscular canal that extends from the cervix to the vulva.

Although there is wide anatomical variation, the length of the unaroused vagina of a woman of child-bearing age is approximately 6 to 7.5 cm (2.5 to 3 in) across the anterior wall (front), and 9 cm (3.5 in) long across the posterior wall (rear). During sexual arousal the vagina expands in both length and width. Its elasticity allows it to stretch during sexual intercourse and during birth to offspring. The vagina connects the superficial vulva to the cervix of the deep uterus.

If the woman stands upright, the vaginal tube points in an upward-backward direction and forms an angle of slightly more than 45 degrees with the uterus. The vaginal opening is at the caudal end of the vulva, behind the opening of the urethra. The upper one-fourth of the vagina is separated from the rectum by the rectouterine pouch. Above the vagina is the Mons pubis. The vagina, along with the inside of the vulva, is reddish pink in color, as are most healthy internal mucous membranes in mammals. A series of ridges produced by folding of the wall of the outer third of the vagina is called the vaginal rugae. They are transverse epithelial ridges and their function is to provide the vagina with increased surface area for extension and stretching.

Vaginal lubrication is provided by the Bartholin's glands near the vaginal opening and the cervix. The membrane of the vaginal wall also produces moisture, although it does not contain any glands. Before and during ovulation, the cervix's mucus glands secretes different variations of mucus, which provides an alkaline environment in the vaginal canal that is favorable to the survival of sperm.

The hymen is a membrane of tissue which is situated at the opening of the vagina. As in many female animals, the hymen covers the opening of the vagina from birth until it is ruptured during sexual or other activity. The tissue may be ruptured by vaginal penetration, delivery, a pelvic examination, injury, or sports. The absence of a hymen does not indicate prior sexual activity, as it is not always ruptured during sexual intercourse. Similarly, its presence does not indicate a lack of prior sexual activity, as light activity may not rupture it, and it can be surgically restored.

Function
The vagina has several biological functions.
Sexual activity
The concentration of the nerve endings that lie close to the entrance of a woman's vagina can provide pleasurable sensation during sexual activity, when stimulated in a way that the particular woman enjoys. During sexual arousal, and particularly the stimulation of the clitoris, the walls of the vagina self-lubricate. This reduces friction that can be caused by various sexual activities. Research has found that portions of the clitoris extend into the vulva and vagina.

With arousal, the vagina lengthens rapidly to an average of about 4 in.(10 cm), but can continue to lengthen in response to pressure. As the woman becomes fully aroused, the vagina tents (last ²⁄3) expands in length and width, while the cervix retracts. The walls of the vagina are composed of soft elastic folds of mucous membrane which stretch or contract (with support from pelvic muscles) to the size of the inserted penis or other object, stimulating the penis and helping to cause the male to experience orgasm and ejaculation, thus enabling fertilization.

The vagina is self-cleansing and therefore usually needs no special treatment. Doctors generally discourage the practice of douching. Since a healthy vagina is colonized by a mutually symbiotic flora of microorganisms that protect its host from disease-causing microbes, any attempt to upset this balance may cause many undesirable outcomes, including but not limited to abnormal discharge and yeast infection. The acidity of a healthy vagina of a woman of child-bearing age (a pH of around 4.5) is due to the degradation of glycogen to the lactic acid by enzymes secreted by the Doderlein's bacillus. This is a normal commensal of the vagina. The acidity retards the growth of many strains of dangerous microbes.

The vagina is examined during gynecological exams, often using a speculum, which holds the vagina open for visual inspection of the cervix or taking of samples

The following measures are advisable for keeping the vulva and vagina healthy:

Washing once a day or so with water but without soap, since soap disturbs the natural pH balance of the vagina. Some, so called pH balanced soaps exist, but their effects remain unclear. Other measures are seldom necessary or advisable. Two notable examples: so-called "feminine hygiene sprays" are unnecessary, may be generally harmful, and have been known to cause severe allergic reactions. Vaginal douching is generally not necessary and has been implicated in helping to cause bacterial vaginosis (BV) and candidiasis ("yeast infections").
After using the toilet, wipe from the front toward the back to avoid introducing bacteria from the anal area into the vulva. Use non-perfumed, undyed toilet paper.
Drink plenty of water and urinate frequently and as soon as possible when you feel the need, to help flush bacteria out of the urinary tract and avoid urinary tract infections. For the same reason, try to urinate before and after sex.
Change out of a wet swimsuit or other wet clothes as soon as possible.
Avoid fragrances, colors, and "deodorants" in products that contact the vulva/vagina: sanitary napkins, tampons, toilet paper. Some women who are sensitive to these substances should also avoid bubble baths and some fabric detergents and softeners.
Avoid wearing leather trousers, tight jeans, panties made of nylon or other synthetic fabrics, or pantyhose without an all-cotton crotch (not cotton covered by nylon - cut out the nylon panel if necessary).
Anything which has been in contact with the anal area (see anal sex) should be thoroughly washed with soap and water or a similar disinfectant before coming in contact with the vulva or vagina.
Use condoms during heterosexual intercourse, practice safer sex, know your sex partners, ask sex partners to practice basic hygiene of their genitals. Use artificial lubrication during the intercourse if the amount naturally produced is not enough.
Be careful with objects inserted inside the vagina. Improper insertion of objects into any body opening can cause damage: infection, cutting, piercing, trauma, blood loss, etc.
Avoid letting any contaminants inside the vagina, including dirt but especially sand.
Women who are unable to walk are more likely to have infections. The problem can be prevented according to above-mentioned and following measures:

Wash crotch and rectal areas (with a soap-substitute if desired) and large amount of warm running water, every morning and evening. The disabled person can sit on a shower chair with an open seat or on a toilet. Use a shower head or water container to wash more directly.
Use towel to dry.
DO NOT use talcum/body powder, if desired use corn starch powder (corn-flour) on the skin of the genital area to absorb perspiration.
Avoid sitting on plastic or synthetic materials for extended lengths of time.
Wear loose underpants and change if soilage or wetness occurs.