noun

• 1Engineering
  historical Each of two heavy beams located on either side of the cylinder in some early steam engines, which are pivoted centrally to oscillate through a small arc and transmit reciprocating motion from the cross head of the pistons to the connecting rods or crankshaft. Frequently attributive.

• 2In a firearm: a lever that is situated alongside the action and operated by the user, typically to allow the barrel to be opened for loading and locked to the action for firing.

• 3Generally. A lever situated at the side of something.