decrement

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The act or process of decreasing or becoming gradually less.
• noun The amount lost by gradual diminution or waste.
• noun Mathematics The amount by which a variable is decreased; a negative increment.

from The Century Dictionary.

• noun The act or state of decreasing; the becoming gradually less; lessening; waste.
• noun The quantity lost by gradual diminution or waste; specifically, in mathematics, the small part by which a variable quantity becomes less and less.
• noun In heraldry, the condition of waning: said of the moon. It is represented by turning the horns of the crescent toward the sinister side. Also called detriment.
• noun In crystallography, a successive diminution of the layers of molecules applied to the faces of the primitive form, by which the secondary forms are hypothetically produced.

from the GNU version of the Collaborative International Dictionary of English.

• noun The state of becoming gradually less; decrease; diminution; waste; loss.
• noun The quantity lost by gradual diminution or waste; -- opposed to increment.
• noun (Crystallog.) A name given by Haüy to the successive diminution of the layers of molecules, applied to the faces of the primitive form, by which he supposed the secondary forms to be produced.
• noun (Math.) The quantity by which a variable is diminished.
• noun The decrease of life in a group of persons in which the assumed law of mortality is such that the ratio of those dying in a year to those living through the year is constant, being independent of the age of the persons.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A small quantity removed or lost. One of a series of regular subtractions.
• verb To decrease a value by a basic quantity unit.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the amount by which something decreases
• noun a process of becoming smaller or shorter