half-ray

from The Century Dictionary.

• noun In mathematics, the aggregate of all points of a straight which are on one and the same side of O, a point of the straight.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Geom.) A straight line considered as drawn from a center to an indefinite distance in one direction, the complete ray being the whole line drawn to an indefinite distance in both directions.

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

• noun geometry A ray, or half-line.