Continental Drift
Continental drift is the hypothesis, proposed by Alfred Wegener in 1912, that the continents were once joined and have since moved apart.
Definition
Continental drift is the idea that the continents were once all stuck together in one giant landmass and have slowly drifted apart to where they are now. It is Alfred Wegener's hypothesis that the continents were joined in a supercontinent, Pangaea, about $300$ million years ago and later split and moved to their present positions, supported by the fit of coastlines, matching rock types and mountain belts across oceans, identical fossils such as Mesosaurus and Glossopteris on separate continents, and glacial deposits in now-tropical regions. Wegener's 1912 hypothesis assembled strong geological, paleontological, and paleoclimatic evidence but proposed physically implausible mechanisms (tidal forces, continents plowing through oceanic crust), which geophysicists such as Harold Jeffreys rightly rejected. The revival came from paleomagnetism in the 1950s, when apparent polar wander paths for different continents matched only if the continents had moved relative to one another, and from Harry Hess's seafloor spreading in 1962, which supplied the mechanism. Continental drift is now a consequence of plate tectonics rather than a separate theory.
Example
Look at a world map: the east coast of South America fits into the west coast of Africa like two puzzle pieces. The Appalachian Mountains line up with the Caledonian Mountains of Scotland and Norway, one continuous range before the Atlantic opened, and coal beds in Antarctica show it once had warm forests. Paleomagnetic data from Triassic red beds in the Deep River basin of North Carolina record the latitude at which they formed, placing the region near $15^\circ$ north when Pangaea began to rift. Matching the $500$-meter depth contour rather than the coastline, Bullard's 1965 computer fit of the Atlantic margins left gaps of under $1$ degree.
Key Insight
Alfred Wegener had the right idea in 1912 but could not explain how continents moved, so most scientists did not believe him for fifty years. Continental drift became plate tectonics once the mechanism was found: continents do not plow through the ocean floor, they ride on plates that include the ocean floor. The rejection of continental drift is a case study in scientific method. The evidence was strong, but without a mechanism consistent with known physics the idea could not be accepted, and once the mechanism appeared the community reversed within a decade.