Circular definition

A circular definition is a recursive definition. For instance, we can define "oak" as a tree which has catkins[?] and grows from an acorn, and then define "acorn" as the nut produced by an oak tree. If someone doesn't know which trees are oaks (or which nuts are acorns) to begin with, they are left wondering.

A circular definition occurred in the definition of the kilogram. The kilogram was originally defined as the mass of one litre of water at standard pressure and the temperature at which it is densest (which is about 4°C). The unit of pressure is the newton per square metre, where a newton is the force that accelerates a kilogram one metre per second squared. Thus the kilogram was defined in terms of itself. It was later defined as the mass of a certain piece of metal in Sèvres.

See also Fallacies of definition.