right ascension

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The angular distance of a celestial body or point on the celestial sphere, measured eastward from the vernal equinox along the celestial equator to the hour circle of the body or point and expressed in degrees or hours.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun astronomy The angular distance east of the vernal equinox point; the celestial equivalent of longitude.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (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