two stars that appear as one if not viewed through a telescope with adequate magnification, such as two stars that are separated by a great distance but are nearly in line with each other and an observer (optical double star) or those that are relatively close together and comprise a single physical system (physical double star)
British Dictionary definitions for optical double star
optical double star
noun
1.
two stars that appear close together when viewed through a telescope but are not physically associated and are often separated by a great distance Compare binary star
double star
noun
1.
two stars, appearing close together when viewed through a telescope; either physically associated (binary star) or not associated (optical double star) See also binary star, optical double star