The twin paradox compares the aging of a traveling twin who makes a near-light-speed journey with that of a twin who remains on Earth. When the traveler returns, the traveling twin has aged less than the stay-at-home twin.
The result follows from proper time: different paths through spacetime can contain different elapsed times. The Earth twin follows one approximately inertial world line, while the traveler changes inertial frames at the turnaround, or more realistically accelerates during the journey. The two paths are therefore not physically symmetrical, despite the fact that each sees the other moving during parts of the trip.
The word “paradox” describes the apparent contradiction created by applying time dilation too naively. It does not signal a logical inconsistency in relativity. The difference can be calculated using special relativity alone; general relativity is not required, although acceleration provides an intuitive way to identify the broken symmetry.
Paul Langevin gave an influential early discussion in 1911. The effect has practical relatives in precision-clock experiments and in satellite navigation, where motion and gravitational differences must be accounted for. The traveling twin’s lower elapsed time is not simply caused by speed alone; the complete path and frame changes matter.