Full Idea
Notable examples of definitions in philosophy have been Plato's (e.g. of piety, in 'Euthyphro'), Anselm's definition of God, the Frege-Russell definition of number, and Tarski's definition of truth.
Gist of Idea
Notable definitions have been of piety (Plato), God (Anselm), number (Frege), and truth (Tarski)
Source
Anil Gupta (Definitions [2008], Intro)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.1
A Reaction
All of these are notable for the extensive metaphysical conclusions which then flow from what seems like a fairly neutral definition. We would expect that if we were defining essences, but not if we were just defining word usage.