PHA's are Potentially Hazardous Asteroids. PHAs are a subclass of near-Earth asteroids (asteroids that approach Earth's orbit) that are defined by two criteria: 1) PHAs are at least 140-meters across (~460 feet), and 2) their orbits bring them within 0.05 AU of Earth's orbit (= 4.6 million miles or 7.5 milliom km). This distance is roughly 20x the Earth-moon distance. Currently there are about 2262 known PHAs, including 157 larger than 1-km. The largest PHA is 7-km across. More information is available on JPL's NEO Basics: NEO Groups page.
Image credit: CSS Orbit View, D. Rankin