Unicellular organism
A unicellular organism, also known as a single-celled organism is an organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

 that consists of only one cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

, in contrast to a multicellular organism
Multicellular organism
Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. Most life that can be seen with the the naked eye is multicellular, as are all animals and land plants.-Evolutionary history:Multicellularity has evolved independently dozens of times...

 that consists of multiple cells. Historically simple single celled organisms have sometimes been referred to as monads

Prokaryotes, most protists, and some kinds of fungi are unicellular. Although some of these organisms live in colonies, they are still unicellular. These organisms live together, and each cell in the colony is the same. However, each cell must carry out all life processes in order for that cell to survive. In contrast, even the simplest multicellular organisms have specialized cells that depend on each other in order to survive.

Most unicellular organisms are of microscopic
The microscopic scale is the scale of size or length used to describe objects smaller than those that can easily be seen by the naked eye and which require a lens or microscope to see them clearly.-History:...

 size and are thus classified as microorganisms. However, some unicellular protists and bacteria are macroscopic
The macroscopic scale is the length scale on which objects or processes are of a size which is measurable and observable by the naked eye.When applied to phenomena and abstract objects, the macroscopic scale describes existence in the world as we perceive it, often in contrast to experiences or...

 and visible to the naked eye. Examples include:
  • Xenophyophore
    Xenophyophores are marine protozoa, giant single-celled organisms found throughout the world's oceans, at depths of up to 10,641 meters . Xenophyophores are found in the greatest numbers on the abyssal plains of the deep ocean. They were first described as sponges in 1889, then as testate...

    s are the largest examples known, with Syringammina fragilissima
    Syringammina fragilissima
    Syringammina fragilissima is a xenophyophore found off the coast of Scotland, near Rockall. It is the largest single-celled organism known, at up to across...

    achieving a diameter of up to 20 cm.

  • Valonia ventricosa
    Valonia ventricosa
    Valonia ventricosa, also known as "bubble algae" and "sailors’ eyeballs", is a species of algae found in oceans throughout the world in tropical and subtropical regions...

    , of the class Chlorophyceae
    The Chlorophyceae are one of the classes of green algae, distinguished mainly on the basis of ultrastructural morphology. For example the chlorophycean CW clade, and chlorophycean DO clade, are defined by the arrangement of their flagella. Members of the CW clade have flagella that are displaced...

    , can achieve a diameter of 1 to 4 cm.
  • Thiomargarita namibiensis
    Thiomargarita namibiensis
    Thiomargarita namibiensis is a gram-negative coccoid Proteobacterium, found in the ocean sediments of the continental shelf of Namibia. It is the largest bacterium ever discovered, in general, wide, but sometimes up to...

    is the largest bacteria
    Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

    , achieving a diameter of up to 0.75 mm.
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