What appears to be asingle star in the sky that, when magnified, is revealed to actually be two or more stars that are very close to each other. These are often only optical doubles, although some are true binary stars.
two stars that lie very close to, and often orbit, each other. Line-of-sight doubles are a consequence of perspective and aren't physically related. Most stars, however, are multiples gravitationally bound together. Usually the stars orbit so closely that they appear as a single point of light even when viewed through professional telescopes.
Two stars that appear near each other in the sky. Their apparent closeness may be due to chance alignment—with one star, in fact, far beyond the other—or they may be in orbit around a common center of gravity, in which case they form a binary star.
A system containing two or more stars. In many double star systems, the stars are not actually physicall close to eachother, they only appear to be beacuse of the way we view the sky. True double stars are called binary stars. See also: binary star, of course.
A double star is two stars that appear close to one another in the sky. Some are true binaries (two stars that revolve around one another); others just appear together from the Earth because they are both in the same line-of-sight.