The American Academy of Dermatology defines Alopecia thus:
Alopecia (al-oh-PEE-shah) means hair loss. When a person has a medical condition called alopecia areata (ar-ee-AH-tah), the hair falls out in round patches. The hair can fall out on the scalp and elsewhere on the body.
Read more on their site: https://www.aad.org/public/diseases/hair-and-scalp-problems/alopecia-areata