'Swim bladder' definitions:

Definition of 'swim bladder'

noun

An air-filled sac near the spinal column in many fishes that helps maintain buoyancy [syn: air bladder, swim bladder, float]

From: GCIDE

Swim \Swim\, n.
1. The act of swimming; a gliding motion, like that of one swimming. --B. Jonson. [1913 Webster]
2. The sound, or air bladder, of a fish. [1913 Webster]
3. A part of a stream much frequented by fish. [Eng.] [1913 Webster]
Swim bladder, an air bladder of a fish.
To be in the swim, to be in a favored position; to be associated with others in active affairs. [Colloq.] [1913 Webster]