A megalith is a large stone that has been used to construct a structure or monument, either alone or together with other stones. The word “megalithic” describes structures made of such large stones without the use of mortar or concrete, representing periods of prehistory characterised by such constructions. For later periods, the term monolith, with an overlapping meaning, is more likely to be used.
The word “megalith” comes from the Ancient Greek “μέγας” (transl. megas meaning “great”) and “λίθος” (transl. lithos meaning “stone”). Megalith also denotes an item consisting of rock(s) hewn in definite shapes for special purposes. It has been used to describe buildings built by people from many parts of the world living in many different periods.
Source — Wikipedia