(1) Holding back within the body or within an organ, tissue or cell of matter that is normally eliminated. Ant: elimination. (2) Holding in memory of what has been learned for later use as recall, recognition or relearning. (3) Amount of a substance that is left from the total absorbed after a certain time following exposure: if the retention follows a course in relation to time that is a first order process, it may be described in terms of biological half-life (half-time). Related term: half-life