A gravitational phenomenon where an orbiting body's rotational period matches its orbital period, causing the same hemisphere to always face the primary body. The Moon is tidally locked to Earth, always showing us the same side.
The term combines 'tidal' from the gravitational effects that cause ocean tides, and 'locking' suggesting a fixed relationship. The concept developed in the 19th century as astronomers understood how gravitational forces could synchronize rotation and orbital motion.
Tidal locking creates some of the most extreme environments in the universe - tidally locked exoplanets have permanent day and night sides, with scorching temperatures on one side and frozen conditions on the other! This creates powerful atmospheric winds and could actually make some tidally locked worlds more habitable by redistributing heat.
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