Endometriosis occurs when tissue like that which lines the inside of uterus grows outside the uterus, usually on the surfaces of organs in the pelvic and abdominal areas, in places that it is not supposed to grow.

The word endometriosis comes from the word “endometrium”—endo means “inside” and metrium (pronounced mee-tree-um) means “mother.” Health care providers call the tissue that lines the inside of the uterus (where a mother carries her baby) the endometrium.

Health care providers may call areas of endometriosis by different names, such as implants, lesions, or nodules.

References

http://www.nichd.nih.gov/publications/pubs/endometriosis/index.htm#contents