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any of various relatively long pieces of metal, wood, stone, etc., manufactured or shaped especially for use as rigid members or parts of structures or machines.
Building Trades. a horizontal bearing member, as a joist or lintel.
Engineering. a rigid member or structure supported at each end, subject to bending stresses from a direction perpendicular to its length.
Nautical.

a horizontal structural member, usually transverse, for supporting the decks and flats of a vessel.
the extreme width of a vessel.
the shank of an anchor.

Aeronautics. the direction perpendicular to the plane of symmetry of an aircraft and outward from the side.
the widest part.
Slang. the measure across both hips or buttocks:
broad in the beam.
Machinery.

walking beam.
(in a loom) a roller or cylinder on which the warp is wound before weaving.
a similar cylinder on which cloth is wound as it is woven.

the crossbar of a balance, from the ends of which the scales or pans are suspended.
a ray of light:
The sun shed its beams upon the vineyard.
a group of nearly parallel rays.
Radio, Aeronautics. a signal transmitted along a narrow course, used to guide pilots through darkness, bad weather, etc.
Electronics. a narrow stream of electrons, as that emitted from the electron gun of a cathode ray tube.
the angle at which a microphone or loudspeaker functions best.
the cone-shaped range of effective use of a microphone or loudspeaker.
Citizens Band Radio Slang. beam antenna.
a gleam; suggestion:
a beam of hope.
a radiant smile.
the principal stem of the antler of a deer.
to emit in or as in beams or rays.
Radio. to transmit (a signal) in a particular direction.
Radio and Television. to direct (a program, commercial message, etc.) to a predetermined audience.
to emit beams, as of light.
to smile radiantly or happily.
beam in, Citizens Band Radio Slang. to be received under optimum conditions; be heard loud and clear:
They told me I was really beaming in.
fly the beam, Radio, Aeronautics. (of an aircraft) to be guided by a beam.
off the beam,

not on the course indicated by a radio beam.
Informal. wrong; incorrect:
The pollsters were off the beam again for the last presidential election.

on the beam,

on the course indicated by a radio beam, as an airplane.
Nautical. at right angles to the keel.
Informal. proceeding well; correct; exact:
Their research is right on the beam and the results should be very valuable.

Contemporary Examples

Back then, no one ever imagined needing to beam live video to ground troops from a fighter jet.
Newest U.S. Stealth Fighter ‘10 Years Behind’ Older Jets Dave Majumdar December 25, 2014

Her door stands ajar, halving the room with a beam of light.
After the Genocide, Rwanda’s Widows Aging Alone Nina Strochlic August 30, 2014

Rather than plunging us into innocent love with an apparent stranger, they beam our conscious self-regard back at ourselves.
In Defense of the Selfie, Oxford English Dictionary’s Word of the Year James Poulos November 19, 2013

After Szabo edged out Retton in both the bars and beam, a shot at a U.S. gold seemed increasingly unlikely.
6 Classic Olympic Tearjerker Moments (Video) Sara Gilford July 26, 2012

Flynn has navigated his way to the MCP in the meantime, and dives into the program’s beam.
Catch Up on Tron: Watch 6 Key Moments Alex Berg December 15, 2010

Historical Examples

A beam, by the way, is a beam in Japan; anything under a foot thick is a stick.
From Sea to Sea Rudyard Kipling

I keep the beam out of my own eye which I have no hope of pulling out of my neighhour’s.
Weighed and Wanting George MacDonald

Luckily for us, the wind was nearly aft, so that we did not feel its effects nearly so much as if it had been on our beam.
A Boy’s Voyage Round the World The Son of Samuel Smiles

The beam of a star must lead any thoughtful soul into endless reveries.
The Cruise of the Dry Dock T. S. Stribling

It was as though the beam were a very fragile thing that might break should it brush even the smallest tree.
The Spell of the White Sturgeon James Arthur Kjelgaard

noun
a long thick straight-sided piece of wood, metal, concrete, etc, esp one used as a horizontal structural member
any rigid member or structure that is loaded transversely
the breadth of a ship or boat taken at its widest part, usually amidships
a ray or column of light, as from a beacon
a broad smile
one of the two cylindrical rollers on a loom, one of which holds the warp threads before weaving, the other the finished work
the main stem of a deer’s antler from which the smaller branches grow
the central shaft of a plough to which all the main parts are attached
a narrow unidirectional flow of electromagnetic radiation or particles: a beam of light, an electron beam
the horizontal centrally pivoted bar in a balance
(informal) the width of the hips (esp in the phrase broad in the beam)
a beam in one’s eye, a fault or grave error greater in oneself than in another person
off beam, off the beam

not following a radio beam to maintain a course
(informal) wrong, mistaken, or irrelevant

on the beam

following a radio beam to maintain a course
(nautical) opposite the beam of a vessel; abeam
(informal) correct, relevant, or appropriate

verb
to send out or radiate (rays of light)
(transitive) to divert or aim (a radio signal or broadcast, light, etc) in a certain direction: to beam a programme to Tokyo
to pass (data, esp business card details, etc) from one hand-held computer to another by means of infrared beams
(intransitive) to smile broadly with pleasure or satisfaction
n.

Old English beam originally “living tree,” but by late 10c. also “rafter, post, ship’s timber,” from Proto-Germanic *baumaz (cf. Old Norse baðmr, Old Frisian bam “tree, gallows, beam,” Middle Dutch boom, Old High German boum, German Baum “tree,” Gothic bagms), perhaps from PIE verb root *bheue- “to grow” (see be). The shift from *-au- to -ea- is regular in Old English.

Meaning “ray of light” developed in Old English, probably because it was used by Bede to render Latin columna lucis, the Biblical “pillar of fire.” Nautical sense of “one of the horizontal transverse timbers holding a ship together” is from early 13c., hence “greatest breadth of a ship,” and slang broad in the beam “wide-hipped” (of persons). To be on the beam (1941) was originally an aviator’s term for “to follow the course indicated by a radio beam.”
v.

“emit rays of light,” early 15c., from beam (n.) in the “ray of light” sense. Sense of “to smile radiantly” is from 1804; that of “to direct radio transmissions” is from 1927. Related: Beamed; beaming.

jargon
(From Star Trek Classic’s “Beam me up, Scotty!”) To transfer softcopy of a file electronically; most often in combining forms such as “beam me a copy” or “beam that over to his site”. Compare blast, snarf, BLT.
[Jargon File]
(2009-06-09)
biology, electronics, aesthetics, and mechanics (robotics)

occurs in the Authorized Version as the rendering of various Hebrew words. In 1 Sam. 17:7, it means a weaver’s frame or principal beam; in Hab. 2:11, a crossbeam or girder; 2 Kings 6:2, 5, a cross-piece or rafter of a house; 1 Kings 7:6, an architectural ornament as a projecting step or moulding; Ezek. 41:25, a thick plank. In the New Testament the word occurs only in Matt. 7:3, 4, 5, and Luke 6:41, 42, where it means (Gr. dokos) a large piece of wood used for building purposes, as contrasted with “mote” (Gr. karphos), a small piece or mere splinter. “Mote” and “beam” became proverbial for little and great faults.

see:

broad in the beam
off the beam