An asteroid with an orbit that is predicted to bring it within about 0.05 au of the Earth (i.e. about 20 times the distance of the Moon) and a size large enough to cause significant damage should it hit the Earth, usually taken as a diameter of at least 150 m. As of 2017 nearly 1800 PHAs were known, but none of these is likely to strike the Earth in the foreseeable future.

http://neo.jpl.nasa.gov/neo/groups.html NASA information page.