The fluid in spaces between the tissue cells, constituting about 16% of the weight of the body; closely similar in composition to LYMPH. (On-Line Medical Dictionary [Internet]. University of Newcastle upon Tyne (UK): The CancerWEB Project, Academic Medical Publishing; c1997-2003 [cited 2003 May 1]. Available from: http://cancerweb.ncl.ac.uk/omd/.)