A planet or moon's habitability is its ability to host life. Factors include:
The latter implies the planet not be tidally locked. Study suggests an atmosphere does much to slow the tidal locking process.
Sun-like stars are considered likely to include habitable extra-solar planets, as are red dwarves. In the latter case, the nearness of the habitable zone makes tidal locking more likely, but if an atmosphere truly decreases the chance of tidal locking, the large number of red dwarfs makes finding a habitable exoplanet among them likely.
A star and planet's history affects its habitability: for example, a star's higher flux at early times (e.g., extreme ultraviolet flux) can affect a planet's atmosphere and its future, which might produce a planet of reasonable size in the habitable zone but without the right sort of atmosphere. Related is the amount of time in the history of the planet that it can be considered habitable, as the chance of life forming depends upon the amount of time that conditions are favorable, as does each level of the complexity of that life.