"hawk-headed sovereign sun god of Egyptian mythology," from Egyptian R' "sun, day."

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Definitions of Ra from WordNet
ra (n.)
(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;
Synonyms: right ascension / celestial longitude
Ra (n.)
an intensely radioactive metallic element that occurs in minute amounts in uranium ores;
Synonyms: radium / atomic number "
Ra (n.)
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;
Synonyms: Re