An imaginary line around which an object rotates. In a rotating sphere, such as the Earth and other planets, the two ends of the axis are called poles. The 23.45° tilt of the Earth's axis with respect to the plane of its orbit around the Sun causes the Northern and Southern Hemispheres to point toward and away from the Sun at different times of the year, creating seasonal patterns of weather and climate. Other planets in the solar system have widely varying tilts to their axes, ranging from near 0° for Mercury to 177° for Venus.
A line, ray, or line segment with respect to which a figure or object is symmetrical.
A reference line from which distances or angles are measured in a coordinate system, such as the x-axis and y-axis in the Cartesian coordinate system.
Anatomy The second cervical vertebra, which serves as a pivot for the head.
Botany The main stem or central part of a plant or plant part, about which other plant parts, such as branches or leaflets, are arranged.