1) A bundle of light rays, or concentrated flow of electromagnetic waves in one direction. 2) A concentrated stream of particles in one direction.
O.E. beam originally "living tree," but by 1000 also "post, ship's timber," from W.Gmc. *baumoz (cf. O.Fris. bam, Du. boom, Ger. Baum "tree"), perhaps from PIE verb root *bu- "to grow." Meaning of "ray of light" developed in O.E., probably because it was used by Bede to render L. columna lucis, Biblical "pillar of fire." The verb meaning "emit rays of light" is from c.1440.