A cromlech (sometimes also spelled "cromleh" or "cromlêh"; cf Welsh crom, "bent"; llech, "slate") is a megalithic construction made of large stone blocks. The word applies to two different megalithic forms in English, the first being an altar tomb (frequently called a "dolmen"), as William Borlase first denoted in 1769. A good example is at Carn Llechart. The second meaning of the name "cromlech" in English refers to large stone circles such as those found among the Carnac stones in Brittany, France.

Unlike in English, the word "cromlech" in many other languages (such as Azerbaijani, Armenian, French, Greek, Indonesian, Italian, and Spanish) exclusively denotes a megalithic stone circle, whereas the word "dolmen" is used to refer to the type of megalithic altar tomb sometimes indicated by the English "cromlech". Also, more recently in English, scholars such as Aubrey Burl use "cromlech" as a synonym for "megalithic stone circle".