A biological model is an organism or system representing a more complex biological entity. It may refer to: a model organism, a non-human species that is extensively studied to understand particular biological phenomena present in many related organisms an in vitro model system, representing complex in vivo systems a mathematical model of a biological system, e.g., the biological neuron model, a mathematical description of the properties of certain cells in the nervous system a scientific model of a biological system, e.g. the fluid mosaic model Models of abnormality#The biological (medical) model, the only model of psychological abnormalities not based on psychological principles