In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbital plane. It differs from orbital inclination. At an obliquity of 0 degrees, the two axes point in the same direction; that is, the rotational axis is perpendicular to the orbital plane.

The rotational axis of Earth, for example, is the imaginary line that passes through both the North Pole and South Pole, whereas the Earth's orbital axis is the line perpendicular to the imaginary plane through which the Earth moves as it revolves around the Sun; the Earth's obliquity or axial tilt is the angle between these two lines. Earth's obliquity oscillates between 22.1 and 24.5 degrees on a 41,000-year cycle. Based on a continuously updated formula (here Laskar, 1986, though since 2006 the IMCCE and the IAU recommend the P03 model), Earth's mean obliquity (without taking into account the nutation in obliquity) is currently about 23°26′10.5″ (or 23.43626°) and decreasing (about 46.806"/Julian century, on average, from 1 January 2023 to 2 January 2123, with Laskar's model, or 46.833"/Julian century, with P03 model). According to P03 astronomical model, Earth's mean obliquity (without taking into account the nutation in obliquity) was 23°26′11.120″ (23.4364172008°) on 1 January 2022, 0 TT, and 23°26′10.634″ (23.4362871883°) on 1 January 2023, 0 TT.

Over the course of an orbital period, the obliquity usually does not change considerably, and the orientation of the axis remains the same relative to the background of stars. This causes one pole to be pointed more toward the Sun on one side of the orbit, and more away from the Sun on the other side—the cause of the seasons on Earth.