Definition 2
Tag 00VZ, permanent. Cite it as [Bourbaki, Tag 00VZ].
Lie Groups and Lie Algebras, chapter VII, § 4. Regular elements of a Lie group
An element of G is said to be regular if it is regular for the adjoint representation of G.
In other words (Prop. 1), an element is regular if, for all elements in a neighbourhood of in G, the dimension of the nilspace of Ad() is equal to the dimension of the nilspace of Ad().
Bourbaki sets the proof directly after the statement, and the corpus does not separate them, so what is above is the statement and its proof together.