Circular definition

A circular definition is a definition that assumes a prior understanding of the term being defined. For example, we can define the "oak" like a tree growing from an acorn, and then we define "acorn" like the nut produced by the oak. To someone who doesn't know which trees are oaks or which nuts are acorns, the definition is of no use.

A circular definition was occupied as an early definition of the kilogram. The kilogram was originally defined as the mass of one liter of water at standard pressure and the temperature at which it is most dense. The unit of pressure is the newton per square meter where one newton is the force accelerating one kilogram one meter per second square. Thus the kilogram was defined in terms of itself. It was later defined as the mass of a certain piece of metal at Sèvres.

A circular definition was also used in the classic definition of death which was "the permanent cessation of the flow of vital bodily fluids," to which one might ask "what does a fluid do? vital?".