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(BIOLOGY) The fundamental unit of terrestrial life. Cells are self-replicating and may exist on their own as independent organisms, as in the case of bacteria, archaea, and protista, or may form colonies or tissues, as in higher life-forms. The two main types of cells are prokaryotic cells and eukaryotic cells.

Each cell consists of protein-rich material that is differentiated into cytoplasm and a nucleus. Forming a boundary around the cytoplasm is a cell membrane, which in plants and some microorganisms is, in turn, surrounded by a cell wall.

Cells were discovered in 1665 by the English scientist Robert Hooke who first observed them through a primitive microscope. Hooke coined the term "cell," in a biological context, when he described the microscopic structure of cork as being like a tiny, bare room or monk's cell.

• CELL BIOLOGY

(MATHEMATICS) (i) A three-dimensional object that is part of a higher-dimensional object, such as a polychoron. A cell is related to higher-dimensional objects in the way that a face, or (two-dimensional) polygon, is related to higher-dimensional objects. For example, a cell is to a 4-dimensional polytope, or polychoron, what a face is to a 3-dimensional polytope, or polyhedron. Often polytopes are classified simply by how many cells they have. For example, the tesseract has eight cells, each one of which is a cube. (ii) The fundamental spatial unit operated on by the rules of a cellular automaton during one generation.