The term failed binary refers to a star system that developed like a binary star except that one of the two young stellar objects did not achieve the mass of a star (currently 75 Jupiter masses is a commonly-used threshold), leaving a brown dwarf or gas giant. The term may be used to refer to an entire system, or may be used as an adjective, or may be used specifically for the lower mass object. The phrase failed binary system more explicitly means the entire system.
The term failed binary is used in discussions of the formation history of star systems and the potential signs and consequences of this history apparent in observation. The term implies primordial binary stars form in a manner differently than a gas giant within a planetary system. Theories now suggest there is more than one way for planets to form, and this might be said to assume that one way begins in the same manner as the formation of a binary star.