Which term describes organisms that consist of a single cell?

The term that refers to organisms composed of a single cell is "unicellular." Unicellular organisms carry out all necessary life functions within that single cell. This includes processes such as metabolism, reproduction, and responding to environmental stimuli. Examples of unicellular organisms include bacteria, archaea, and many protists.

In contrast, multicellular organisms consist of many cells that may have specialized functions and work together to maintain the life of the organism as a whole. Colonial organisms, while they might appear to be composed of multiple cells, are collections of genetically identical cells that may function more independently than in true multicellular organisms. Lastly, acellular pertains to entities not composed of cells, such as viruses, which do not perform life processes on their own and rely on host cells for replication.