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 characterized by such constructions. For later periods, the word "monolith", with an overlapping meaning, is more likely to be used.

The term was first used in reference to Stonehenge by Algernon Herbert in 1849. A variety of large stones are seen as megaliths, with the most widely known megaliths not being tombs. The construction of these structures took place mainly in the Neolithic period (though earlier Mesolithic examples are known) and continued into the Chalcolithic period and the Bronze Age.

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