What it is
E913 is lanolin: an oily, waxy substance secreted by the sebaceous glands of sheep, recovered from raw wool during processing. Chemically a wax ester, not a fat.
Why it matters
Most often used as a moisturiser in cosmetics (lip balms, hand creams, nipple balms) and as a base in pharmaceutical ointments. Also a starting material for animal-derived vitamin D3 (cholecalciferol) — most non-vegan supplements use lanolin-sourced D3.
What to do
Always not vegan. Plant-sourced vitamin D3 is now widely available — derived from lichen. For cosmetics, look for products explicitly labelled vegan; many "natural" cosmetics still contain lanolin.