A vessel's beam refers to its width at its widest point. Example: The beam of the ship is 40 feet.
It can also refer to an object in relation to the beam of the ship. Example: Enemy sail in sight on the weather beam.
An object can also be referred to as 'abeam'. Example: The wind is abeam, sir.
A ship lying on its side is said to be on her 'beam ends'.
- Beam (nautical) at Wikipedia.