Full Idea
With Gentzen's natural deduction, we may say that the introductions represent, as it were, the 'definitions' of the logical constants. The introductions are not literally understood as 'definitions'.
Gist of Idea
Natural deduction introduction rules may represent 'definitions' of logical connectives
Source
Dag Prawitz (Gentzen's Analysis of First-Order Proofs [1974], 2.2.2)
Book Reference
'A Philosophical Companion to First-Order Logic', ed/tr. Hughes,R.I.G. [Hackett 1993], p.210
A Reaction
[Hacking, in 'What is Logic? §9' says Gentzen had the idea that his rules actually define the constants; not sure if Prawitz and Hacking are disagreeing]