What it is
Stearic acid is a common saturated fatty acid, the same waxy molecule that makes beef fat and cocoa butter firm at room temperature. Magnesium stearate is simply stearic acid joined to magnesium, and it earns its keep as a lubricant, stopping powders from clumping and helping tablets slide cleanly out of their moulds. Both are manufactured by breaking fats and oils down into their component fatty acids and then separating out the stearic acid, which is where the vegan question begins. European safety reviewers state plainly that these fatty acids are 'produced by hydrolysis of common animal and vegetable fats and oils', and that stearic acid in particular is sourced 'from animal fats and oils, as well as some vegetable oils'. The animal version has historically meant tallow, the rendered fat of cattle or sheep, while the plant version usually means palm or another vegetable oil.
How to tell
You will meet these two most often in the supplement aisle, because magnesium stearate is the standard lubricant in tablets and capsules, and again in boiled and chewy sweets, chewing gum, the dusting that stops bakery dough sticking, and in soap, deodorant and cosmetics. On a label they read as 'stearic acid', 'magnesium stearate' or the code E570, none of which reveal whether the fat was a plant or an animal. The reassuring news is that most magnesium stearate made today is vegetable-derived, commonly from palm oil, and supplement brands increasingly say so with wording like 'vegetable magnesium stearate' or 'from a vegetable source'. When the label carries that qualifier you are on safe ground; when it just says 'magnesium stearate' the honest answer is that you cannot be sure, and a note to the manufacturer is the only way to confirm. One extra wrinkle for vegans who also avoid palm oil: the plant-derived version is very often palm, so 'not from tallow' and 'palm-free' are two different questions worth asking together.