Definitions

1. adjective

relating to or concerned with a period of time that is roughly equal to the time it takes for the Earth to rotate once on its axis relative to the Sun

“The subsynodic month of a planet is the time it takes for the planet to return to the same position in its orbit relative to the Sun.”

2. noun

a period of time that is roughly equal to the time it takes for the Earth to rotate once on its axis relative to the Sun

“The subsynodic month of Mars is approximately 24.6 Earth days.”