One of the twelve portions into which the year is divided; the twelfth part of a year, corresponding nearly to the length of a synodic revolution of the moon, -- whence the name. In popular use, a period of four weeks is often called a month.
The period of time it takes the Moon to orbit Earth (or any moon to orbit its parent planet). A sidereal month—27 1/3 days—is the time it takes the Moon to orbit Earth and return to the same position relative to the stars. A synodic month—29 1/2 days— is the time it takes the Moon to orbit Earth and return to the same position relative to the Sun and so is the time between a new moon and the full moon.
The period of the revolution of the Moon around the Earth. The month is designated as siderial, tropical, anomalistic, nodical, or synodical according to whether the revolution is relative to a fixed star, vernal equinox, perigee, ascending node, or Sun. The calendar month is a rough approximation to the synodical month.