A variation of lambda notation for specifying functions using numbers instead of names to refer to formal parameters. A reference to a formal parameter is a number which gives the number of lambdas (written as \ here) between the reference and the lambda which binds the parameter. E.g. the function \ f . \ x . f x would be written \ . \ . 1 0. The 0 refers to the innermost lambda, the 1 to the next etc. The chief advantage of this notation is that it avoids the possibility of name capture and removes the need for alpha conversion.
[N.G. De Bruijn, "Lambda Calculus Notation with Nameless Dummies: A Tool for Automatic Formula Manipulation, with Application to the Church-Rosser Theorem", Indag Math. 34, pp 381-392].