Fatty lubricant matter secreted by sebaceous glands, and made of made of triglyceride oils, wax, squalene, and metabolytes of fat-producing cells[BTO,WP].
Comment:
[function-note] "waterproofs skin"; [taxon-note] "The composition of sebum varies across species. In humans, the lipid content is as follows: 25% wax monoesters, 41% triglycerides, 16% free fatty acids, 12% squalene; Sapienic acid is a sebum fatty acid that is unique to humans."