Mature cystic teratoma is a very common benign ovarian lesion, also called dermoid cyst. 
Mature in this context means benign, as opposed to the immature, malignant teratoma. 
Benign cystic teratomas typically occur in young women of child-bearing age. 
At imaging they are usually unilocular (up to 90%) but can be multilocular, and are bilateral in ~15%. 
Up to 60% may contain calcifications. 
The cystic component is fluid fat, produced by sebaceous glands.