The tropics are the region of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. Because of Earth's axial tilt, the width of the tropics (in latitude) is twice the tilt. The tropics are also referred to as the tropical zone and the torrid zone (see geographical zone).

Due to the sun's high angle throughout the year, the tropics receive the most solar energy over the course of the year, and consequently have the highest temperatures on the planet. Even when not directly overhead, the sun is still close to overhead throughout the year, therefore the tropics also have the lowest seasonal variation on the planet; "winter" and "summer" lose their temperature contrast. Instead, seasons are more commonly divided by precipitation variations than by temperature variations or daylight hours.

The tropics maintain wide diversity of local climates, such as rain forests, monsoons, savannahs, deserts, and high altitude snow-capped mountains. The word "tropical" can specifically refer to certain kinds of weather, rather than to the geographic region; these usages ought not be confused.

The Earth's axial tilt is currently around 23.4°, and therefore so are the latitudes of the tropical circles, marking the boundary of the tropics: specifically, ±23°26′09.3″ (or 23.43593°). The northern one is called the Tropic of Cancer, and the southern is the Tropic of Capricorn. As the Earth's axial tilt changes, so too do the tropical and polar circles.

The tropics constitute 39.8% of Earth's surface area and contain 36% of Earth's landmass. As of 2014^[update], the region was home also to 40% of the world's population, and this figure was then projected to reach 50% by 2050. Because of global warming, the weather conditions of the tropics are expanding with areas in the subtropics, having more extreme weather events such as heatwaves and more intense storms. These changes in weather conditions may make certain parts of the tropics uninhabitable.

The word "tropic" comes via Latin from Ancient Greek τροπή (tropē), meaning "to turn" or "change direction".

The tropics are defined as the region between the Tropic of Cancer in the Northern Hemisphere at 23°26′09.3″ (or 23.43593°) N and the Tropic of Capricorn in the Southern Hemisphere at 23°26′09.3″ (or 23.43593°) S; these latitudes correspond to the axial tilt of the Earth.

The Tropic of Cancer is the Northernmost latitude from which the Sun can ever be seen directly overhead, and the Tropic of Capricorn is the Southernmost. This means that the tropical zone includes everywhere on Earth which is a subsolar point at least once during the solar year. Thus the maximum latitudes of the tropics have equal distances from the equator on either side. Likewise, they approximate the angle of the Earth's axial tilt. This angle is not perfectly fixed, mainly due to the influence of the moon, but the limits of the tropics are a geographic convention, and their variance from the true latitudes is very small.