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A contact binary is a binary star in which the companions' orbit is so small (compared to the size of the companions) that they are in contact. Those that are identified are generally eclipsing binaries, termed W Ursae Majoris variables or W Ursae Majoris contact systems, also a type of variable star.
A binary star that is not a contact binary is a detached binary (wholly separate) or semidetached binary (where one of the two is highly effected, specifically, just one of the two has filled its Roche lobe). Binaries that are not fully detached are additionally termed close binaries or interacting binaries.