Facial hair that grows in the space between (and sometimes past) one's ears and cheeks.

Many reliable sources say that the word sideburns comes from Ambrose E. Burnside, a semi-successful Union General during the Civil War and eventually a U.S. senator from Rhode Island, who made it fashionable to wear long side whiskers.

In parts of Britain, sideburns are also referred to as "Buggers' Grips", (for reasons I'll decline to expand upon).