ratchet

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A mechanism consisting of a pawl that engages the sloping teeth of a wheel or bar, permitting motion in one direction only.
2. n. The pawl, wheel, or bar of this mechanism.
3. v. To cause to increase or decrease by increments: "Some companies . . . may make things worse if they seek to ratchet down their medical expenses by limiting benefits for psychological or psychiatric care” ( Newsweek).
4. v. To increase or decrease by increments.

Century Dictionary and Cyclopedia

1. n. A detent or pivoted piece designed to fit into the teeth of a ratchet-wheel, permitting the wheel to rotate in one direction, but not in the other. A similar device so arranged as to move the wheel is termed a pallet. (See ratchet-wheel click, 3, pawl, and detent.) Combined with the ratchet-wheel as a means of converting a reciprocating into a rotary motion, the ratchet appears in a number of tools and gives its name to each: as, the ratchet bed-key, etc.
2. n. In printing, a notched straight blade of brass which rotates the pinions attached to the movable clamps of an electrotype plate mounted upon a block.

Wiktionary

1. n. A pawl, click, or detent for holding or propelling a ratchet wheel, or ratch, etc.
2. n. A mechanism composed of a ratchet wheel, or ratch and pawl.
3. n. A ratchet wrench.
4. n. A procedure or regulation that goes in one direction, usually up.
5. v. transitive To cause to become incremented or decremented.
6. v. intransitive To increment or decrement.

GNU Webster's 1913

1. n. A pawl, click, or detent, for holding or propelling a ratchet wheel, or ratch, etc.
2. n. A mechanism composed of a ratchet wheel, or ratch, and pawl. See Ratchet wheel, below, and 2d Ratch.

WordNet 3.0

1. v. move by degrees in one direction only
2. n. mechanical device consisting of a toothed wheel or rack engaged with a pawl that permits it to move in only one direction