(1) Having or consisting of only one cell.
(2) Pertaining to an organism whose functions are all carried out within one cell.
Examples of organisms that are unicellular are microorganisms such as bacteria and some algae such as diatoms.
Word origin: L ūni- comb. form of ūnus one + NL cellulāris, equiv. to cellul(a) live cell.
Related forms: unicellularity (noun)