Definition of

Ra

1. (noun, location) (astronomy) the equatorial coordinate specifying the angle, measured eastward along the celestial equator, from the vernal equinox to the intersection of the hour circle that passes through an object in the sky; usually expressed in hours and minutes and seconds; used with declination to specify positions on the celestial sphere
2. (noun, person) ancient Egyptian sun god with the head of a hawk; a universal creator; he merged with the god Amen as Amen-Ra to become the king of the gods
3. (noun, substance) an intensely radioactive metallic element that occurs in minute amounts in uranium ores

via WordNet, Princeton University