The basic answer is: a large stone that is part of a prehistoric structure or monument. The famous pyramids of Giza in Egypt are considered megalithic structures because they were built with enormous blocks of granite or other dense and heavy stone. Stonehenge is also a megalithic site, along with dozens more around the world. The Easter Island statues are also megaliths.
They were built to last and endure the passage of time. They did a good job of that too, because they outlived their civilizations by far, leaving only questions and mysteries as to their purpose and even with which technology they were built.
Technology? Yes - look at this:
This was found in Peru, the home of the ancient Incas who were megalith builders. It looks like the stone was softened so that it could fit into the slot of the other stone. Softening stone for this purpose was something I came across when reading all kinds of books as research for my RHUNA series which is based on theories of an ancient megalithic civilization that spread across the world - perhaps from a legendary advanced place like Atlantis.
One of the most outstanding main features of the megalithic culture around the world is the way stone blocks were cut and put together in a wall. The faces of each block are perfectly straight and smooth, and fit perfectly next to adjoining stone blocks.
Megaliths come in all shapes and sizes, all over the world. Here are some I came across recently:
"Arsh Bilqis" - part of a temple in Yemen.
They don't come in basic rectangles either, but often have many more facets, some with slots, grooves, holes and other mysterious bits which once served to connect the block to another block or device. Check these out!
Peru
Shrine of Panoias, Portugal
A strange megalith with precise cuts in Indonesia...and something even more bizarre in India
And here is a wall in India, identical to those in Cusco, Peru, Easter Island and other megalithic sites:
Warangal, India
And this was just a sample of the extraordinary megaliths around the world!
