The phrase "galactic collision" refers to when two galaxies, which are large collections of stars, gas, and dust, crash into each other. It's like two massive cosmic objects colliding in space. Full definition
These include filamentlike streams of stars — remnants of galactic collisions — as well as small «satellite» galaxies orbiting the Milky Way. (sciencemag.org)
Dawson, Bullock and colleagues are following up with about 20 more galactic collisions to see if they show any unusual behaviour. (newscientist.com)
Psychedelic pictures of 30 galactic collisions show for the first time that merging galaxies often spawn disc - shaped offspring like our Milky Way (newscientist.com)