To become less; lessen; be diminished gradually in extent, bulk, quantity, or amount, or in strength, influence, or excellence: as, the days decrease in length from June to December.To make less; lessen; make smaller in dimensions, amount, quality, excellence, etc.; reduce gradually or by small deductions.n. A becoming less; diminution; wane (as applied to the moon); decay: as, a rapid decrease of revenue or of strength.n. The amount by which something is lessened; extent of loss or decrement: as, a great decrease in production or of income.