Hyperplasia (or "hypergenesis") means increase in number of cells/proliferation of cells. It may result in the gross enlargement of an organ and the term is sometimes mixed with benign neoplasia / benign tumor.

Hyperplasia is a common preneoplastic response to stimulus. Microscopically cells resemble normal cells but are increased in numbers. Sometimes cells may also be increased in size (hypertrophy). Hyperplasia is different from hypertrophy in that the adaptive cell change in hypertrophy is an increase in cell size, whereas hyperplasia involves an increase in the number of cells.

Hyperplasia is considered to be a physiological (normal) response to a specific stimulus, and the cells of a hyperplastic growth remain subject to normal regulatory control mechanisms. This stands in contrast to neoplasia (the process underlying cancer and benign tumors), in which genetically abnormal cells proliferate in a non-physiological manner which is unresponsive to normal stimuli.

As seen in examples below, such physiological proliferation of cells may in fact be secondarily due to a pathological cause. Still, the proliferation itself is a normal response to another abnormal condition, in contrast to neoplasia, where the proliferation in itself is abnormal.