noun, the third planet from the sun, the only planet on which life is known to exist. It is not quite spherical, being flattened at the poles, and consists of three geological zones, the core, mantle, and thin outer crust. The surface, covered with large areas of water, is enveloped by an atmosphere principally of nitrogen (78 per cent), oxygen (21 per cent), and some water vapour. The age is estimated at over four thousand million years. Distance from sun: 149.6 million km; equatorial diameter: 12 756 km; mass: 5.976 ✕ 10²⁴ kg; sidereal period of axial rotation: 23 hours 56 minutes 4 seconds; sidereal period of revolution about sun: 365.256 days. Related adjs terrestrial, tellurian, telluric, terrene

From Collins

©HarperCollins 2008. Collins English Dictionary definition of earth