One that actively contributes to an accomplishment, result, or process: "Surprise is the greatest factor in war\u201d ( Tom Clancy). See Synonyms at element. One who acts for someone else; an agent. A person or firm that accepts accounts receivable as security for short-term loans. Mathematics One of two or more quantities that divides a given quantity without a remainder. For example, 2 and 3 are factors of 6; a and b are factors of ab. A quantity by which a stated quantity is multiplied or divided, so as to indicate an increase or decrease in a measurement: The rate increased by a factor of ten. A gene. No longer in technical usage. Physiology A substance that functions in a specific biochemical reaction or bodily process, such as blood coagulation. To determine or indicate explicitly the factors of. factor in To figure in: We factored sick days and vacations in when we prepared the work schedule.