Origin: 1350–1400; ME
decres (n.),
decresen (v.) < OF
decreiss-, long s. of
decreistre < L
dēcrēscere (
dē- de- +
crēscere to grow);
see crescent 
Synonyms:
1. wane, lessen, fall off, decline, contract, abate. Decrease, diminish, dwindle, shrink imply becoming smaller or less in amount. Decrease commonly implies a sustained reduction in stages, esp. of bulk, size, volume, or quantity, often from some imperceptible cause or inherent process: The swelling decreased daily. Diminish usually implies the action of some external cause that keeps taking away: Disease caused the number of troops to diminish steadily. Dwindle implies an undesirable reduction by degrees, resulting in attenuation: His followers dwindled to a mere handful. Shrink esp. implies contraction through an inherent property under specific conditions: Many fabrics shrink in hot water. 3. abatement, decline, subsidence, shrinking, dwindling, ebbing.
Antonyms:
1. increase, expand.