Unicellular organism

Definition

A unicellular organism or single-celled organism (or single-cell organism) is an organism that comprises only one cell.

They are contrasted with multicellular organisms.

Interaction with existing classifications

• All prokaryotes are unicellular organisms. Prokaryotes include all organisms in the domains archaea and bacteria.
• Among eukaryotes (i.e., the eukarya domain), there are some unicellular organisms and some multicellular organisms.