unicellular organism (noun, pl. unicellular organisms) – organisms that are made of isolated, single cells, i. e., cells that do not organized to communicate with one another sharing different functions. The boundary between unicellular and multicellular organisms is rather fluent. All multicellular organisms show stages when they occur as a single cell. Many single-celled organisms, even prokaryotic ones, often show cell organisation, communication and the division of separate functions between different cells (e. g., the heterocytes of cyanobacteria). Thus, these single cells are not necessarily isolated, but they often act together like truly multicellular organism. Kind of:cell organization-type.