Hair loss, also known as alopecia or baldness, refers to a loss of hair from the head or the body.
Hair loss and hypotrichosis (abnormal hair patterns, predominantly loss or reduction) have many causes including androgenetic alopecia, fungal infection, trauma, radiotherapy, chemotherapy, nutritional deficiencies, and autoimmune diseases.
Hair loss severity occurs across a spectrum with extreme examples including alopecia totalis (total loss of hair on the head) and alopecia universalis (total loss of all hair on the head and body).

Adapted from Wikipedia, the free encyclopedia. Internet. Accessed on December 18, 2015.