A single cell organism is known as an unicellular organism. This type of organism carries out its entire life cycle as one cell, as opposed to a multi-cellular organism, which is composed of many cells.
The life span of an unicellular organism is not as long as a multi-cellular organism. This is due to unicellular only existing with one cell to perform all the duties needed for survival. Damage to the one celled organism can be fatal.
Unicellular organisms are small in size, due to only being made of one cell, making unicellular organisms too small to be viewed by the naked eye. Examples of unicellular organisms include amoeba and bacteria.