Full Idea
We use the symbol 'ι' (Greek 'iota') to form definite descriptions, reading (ιx)F(x) as 'the x which is such that F(x)', or simply as 'the F'.
Gist of Idea
The symbol 'ι' forms definite descriptions; (ιx)F(x) says 'the x which is such that F(x)'
Source
Graeme Forbes (The Metaphysics of Modality [1985], 4.1)
Book Reference
Forbes,Graeme: 'The Metaphysics of Modality' [OUP 1985], p.72
A Reaction
Compare the lambda operator in modal logic, which picks out predicates from similar formulae.