Asteroids which have an orbit that crosses the orbit of the Earth and have a period longer than 1 year. They have a semimajor axes greater than 1 astronomical unit (au), and perihelion distances less than 1.017 au.
The Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo, the first asteroid of this group to be discovered by Karl Wilhelm Reinmuth. They are Earth-crosser asteroids that have orbital semi-major axes greater than that of the Earth. Some can get very close to the Earth, making them a potential threat to our planet (the closer their semi-major axis is to Earth's, the less eccentricity is needed for the orbits to cross).