"at right angles to the keel" of a ship, hence in line with its beam, 1826, nautical, literally "on beam;" see a- (1) + beam (n.) in the nautical sense.

(adv) at right angles to the length of a ship or airplane

At right angles to the keel of the boat, but not on the boat.

At right angles to the keel of the boat, but not on the boat.

A position where an aircraft, object or fix is approximately 90 degrees to the left or right of an aircraft's track.

The bearing of an object 90 degrees from ahead (in a line with the middle of the ship).

At right angles to the ship's length.

•  Alongside, near amidships.

On the beam, a relative bearing at right angles to the centerline of the ship's keel.

On the beam, a relative bearing at right angles to the centerline of the ship's keel.

At right angles to the keel of the boat, but not on the boat.

The bearing of an object 90 degrees from ahead (in a line with the middle of the ship).

On the beam, a relative bearing at right angles to the centerline of the ship's keel.

  On a relative bearing of 90 degrees (abeam to starboard) or 270 degrees (abeam to port).    

At right angles to, or beside a ship.

At right angles to the fore and aft line of the boat, or beside, the boat; on the beam; also Abreast.

 A relative direction that is always at a right angle to the vessels direction of travel.

at right angles to the centerline of, and outside of, the ship. An object that is abeam of your ship is directly off to its side.

'On the beam', a relative bearing at right angles to the centerline of the ship's keel.

Abreast of or at right angles to the fore and aft line of the vessel

on a line at right angles to a ship's or aircraft's length

At a right angle to the length of the boat

On the beam

On the beam, a relative bearing at right angles to the centerline of the ship's keel.

at right angles to, or beside the boat

At right angles to the the fore and aft line of the boat, or beside, the boat; on the beam; also Abreast.

At right angles to the keel of the boat, but not on the boat.

In a line approximately at right angle to the ship’s keel opposite the waist or middle part of a ship. See also BROAD ON THE BEAM.

In Navy talk, adjacent to, not fore or aft, but toward your 9 o'clock or 3 o'clock.. The car that's doorhandle to doorhandle with you on the freeway is abeam. As in, "Damn, honey! [..]

Fix or point that is ninety degrees right or left of the aircraft's position

An aircraft is "abeam" a fix, point, or object when that fix, point, or object is approximately 90 degrees to the right or left of the aircraft track. Abeam indicates a general position rath [..]

To the right, or at right angles to the center of a craft.

(nautical,aircraft) Alongside or abreast; opposite the center of the side of the ship or aircraft. Mid 19^th century. * year=2005|author=William Thomas Generous|title=Swee [..]

lang=en 1800s=1851 * '''1851''' — . ''''. *: Squaring her yards, she bore down, ranged abeam under the Pequod's lee, and lowered a boat; it soon drew nigh; but, as the side-ladder was being rigged [..]

on the side of a ship which is perpendicular to the ships fore and aft line

Alongside, near amidships.

Area on either side of a ship near the mid section.