Full Idea
Many definitions have three elements: the term that is defined, an expression containing the defined term (the 'definiendum'), and another expression (the 'definiens') that is equated by the definition with this expression.
Gist of Idea
Definitions usually have a term, a 'definiendum' containing the term, and a defining 'definiens'
Source
Anil Gupta (Definitions [2008], 2)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.6
A Reaction
He notes that the definiendum and the definiens are assumed to be in the 'same logical category', which is a right can of worms.