The term binary companion refers to a member-star of a binary star.
The term clearly applies to such stars, often meant include
stellar remnants and/or brown dwarfs but generally not to
a companion that has been established to be an extra-solar planet.
The term candidate companion is used for possible such astronomical objects
for which evidence is considered inconclusive regarding the
existence of some companion and/or whether it qualifies as a star.
Note that the term binary companion is also seen applying to the
companion of a planet or minor planet. The term has been
used for solar system objects that are co-orbiting an object
of similar size.