Synonyms: decrease, lessen, reduce, dwindle, abate, diminish, subside. These verbs mean to become smaller or less or to cause something to become smaller or less.erb. 1. decrease-decrease in size, extent, or range; The amount of homework decreased towards the end of the semester; The cabin pressure fell dramatically; her weight fell to under a hundred pounds; his voice fell to a whisper. diminish, lessen, fall.