A body is the physical material of a person or organism. It is only used for organisms which are in one part or whole. There are organisms which change from single cells to whole organisms: for example, slime moulds. For them the term 'body' would mean the multicellular stage. Other uses:
Cell body: here it may be used for cells like neurons which have long axons (nerve fibres). The cell body is the part with the nucleus in it.
The body of a dead person is also called a corpse or cadaver. The dead bodies of vertebrate animals and insects are sometimes called carcasses.