A liniment is a balm or lotion you rub into your skin to soothe your achy muscles. If you're sore after playing basketball all afternoon, you should put some liniment on your legs.
Liniment can take various forms, from an oily lotion to a thick, waxy salve. Strictly speaking, a liniment is medicated and brings a feeling of heat to the skin, easing cramps and stiffness of the underlying muscles. The word was once a technical medical term, but today it's more likely to describe a homemade remedy. The Latin root of liniment is linere, "to smear."