turing machines

finite automata

context-free grammar

A formal grammar whose production rules are of the form:
A -> a

With A a single non-terminal symbol, and a a string of terminals and/or non-terminals (a can be empty). A formal grammar is "context-free" if its production rules can be applied regardless of the context of a non-terminal. No matter which symbols surround it, the single non-terminal on the left hand side can always be replaced by the right hand side. This is what distinguishes context-sensitive grammar.

concrete syntax tree (parse tree)

An ordered, rooted tree that represents the syntactic structure of a string according to some context-free grammar. The term parse tree is used in computational linguistics; in theoretical syntax, the term syntax tree is more common.