A "rare event" refers to something that happens very infrequently or with a low probability. It is an occurrence or situation that is not common or unusual. Full definition
The joint statement — an extremely rare event in French science — was signed by the national academies of agriculture, medicine, pharmacy, sciences, technology and veterinary studies. (dotearth.blogs.nytimes.com)
The volume of collisions will allow scientists to study very rare events with unprecedented precision. (sciencedaily.com)