<Tr> <Th_colspan="2"> Unicellular organism </Th> </Tr> <Tr> <Td_colspan="2"> Valonia ventricosa, a species of alga with a diameter that ranges typically from 1 to 4 centimetres (0.39 to 1.57 in) is among the largest unicellular species </Td> </Tr> <P> A unicellular organism, also known as a single - celled organism, is an organism that consists of only one cell, unlike a multicellular organism that consists of more than one cell . Historically, simple unicellular organisms have been referred to as monads, though this term is also used more specifically to describe organisms of the genus Monas and similar flagellate ameboids . The main groups of unicellular organisms are bacteria, archaea, protozoa, unicellular algae, and unicellular fungi . Unicellular organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms . Unicellular organisms are thought to be the oldest form of life, with early protocells possibly emerging 3.8--4 billion years ago . </P> <P> Prokaryotes, most Protista, and some fungi are unicellular . Although some of these organisms live in colonies, they don't exhibit specialization . These organisms live together, and each cell in the colony is the same . However, each individual cell must carry out all life processes to survive . In contrast, even the simplest multicellular organisms have cells that depend on each other to survive . </P>

Microscopic organism that could be prokaryotic or eukaryotic in nature