A tumulus is a big mound of earth that ancient people piled up over a grave. Someone important died, they were buried, and then earth and stones were heaped on top until there was a small hill standing in the landscape. Thousands of years later the hill is still there, and it is often the only thing left to show that anyone was ever buried on that spot. This is a word from archaeology and history books, not from conversation. You meet it when reading about ancient Britain, ancient Europe, Korea, or Japan — anywhere people built these mounds. Everyday English speakers are more likely to say burial mound, which means the same thing and sounds much less academic. In Britain people also say barrow for the same kind of monument. Two things about the form of the word are worth knowing. The plural is tumuli, not "tumuluses" — it keeps its Latin ending. And on British maps, the word TUMULUS printed in old-style lettering marks exactly this: an ancient burial mound out in a field, sometimes so worn down by centuries of ploughing that you would walk past it thinking it was just a bump in the ground.

古墳、墳丘（考古用語）。就是古代人在墓上堆起來的土丘，台灣的歷史課本講日本的「古墳時代」用的就是這個概念。一般講話不會用這個字，會說「土堆」或「古代的墳」。要注意它不是中文的「墳墓」那種平常掃墓去的墓，而是考古現場那種一座小山般的遺跡。