The term [perineum] has two definitions:

1. It is the area of the trunk inferior to the pelvic diaphragm. As such, the perineum contains the ischioanal fossa, the urogenital diaphragm, and the superficial genitalia.

2. It is the area of the body between the upper thighs containing the external openings of urethra, vagina, and anus. This area is delimited (see image) by the symphysis pubis, ischial tuberosities, and coccyx.

The perineum, as described in the second definition, is formed by two large triangular regions. The anterior region (in purple) is called the urogenital triangle, and the posterior region (in yellow) is called the anal triangle. 

Image property of: CAA.Inc.. Artist: D.M. Klein.
Word suggested and edited by: Dr. Sanford S. Osher , MTD Contributor

Perineum, inferior view