From The Collaborative International Dictionary of English v.0.44 [gcide]:
2. That on or against which a body abuts or presses; as (a) (Arch.) The solid part of a pier or wall, etc., which receives the thrust or lateral pressure of an arch, vault, or strut. --Gwilt. (b) (Mech.) A fixed point or surface from which resistance or reaction is obtained, as the cylinder head of a steam engine, the fulcrum of a lever, etc. (c) In breech-loading firearms, the block behind the barrel which receives the pressure due to recoil.
From WordNet (r) 2.0 [wn]:
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