About: Leaf language

In computational complexity theory, a leaf language is a method of characterizing a complexity class by formalizing what it means for a machine to "accept" an input. Several complexity classes are typically defined in terms of a polynomial-time nondeterministic Turing machine, where each branch can either accept or reject, and the entire machine accepts or rejects as some function of the branches' conditions. For example, a non-deterministic Turing machine accepts if at least one branch accepts, and rejects only if all branches reject. A co-non-deterministic Turing machine, on the other hand, accepts only if all branches accept, and rejects if any branch rejects. Many classes can be defined in this fashion.