A mathematical function used in the statistical description of galaxy clustering. It measures the excess probability of finding a galaxy at a given distance from a particular galaxy, compared with what one would expect if galaxies were distributed randomly throughout space. A positive value of the function indicates that there are more pairs of galaxies with a given separation than there would be in a random distribution, while a negative value indicates that galaxies tend to avoid each other. Observations show that galaxies are highly clustered on scales up to several tens of millions of light years.