From The Art and Popular Culture Encyclopedia
A tumulus (plural tumuli) is a mound of earth and stones raised over a grave or graves. Tumuli are also known as barrows, burial mounds, Hügelgrab or kurgans, and can be found throughout much of the world. A cairn (a mound of stones built for various purposes), might also be originally a tumulus. A long barrow is a long tumulus, usually for numbers of burials.
The word tumulus is Latin for 'mound' or 'small hill', from the PIE root *teuh2- with extended zero grade *tum-, 'to bulge, swell' also found in tumor, thumb, thigh and thousand.