The phrase "hole mergers" refers to a situation where two or more holes (empty spaces or gaps) combine or come together to form a larger hole. Full definition
Finding many black hole mergers in the next few years will be a strong indicator that black holes are not few and far between but many and close together. (lindau-nobel.org)
We'll need to see more black hole mergers before we can tell, though — the signal doesn't give a clear answer either way. (newscientist.com)
The observed signal matches what physicists expected from a black hole merger almost perfectly. (sciencenews.org)