That part of a multicellular organism outside the cells proper, usually taken to be outside the plasma membranes, and occupied by fluid.
Source:ISBN:0198547684
Comment
Note that for multicellular organisms, the extracellular space refers to everything outside a cell, but still within the organism (excluding the extracellular matrix). Gene products from a multi-cellular organism that are secreted from a cell into the interstitial fluid or blood can therefore be annotated to this term.