Indexed languages are a class of formal languages discovered by Alfred Aho;[1] they are described by indexed grammars and can be recognized by nested stack automata.
[1] They qualify as an abstract family of languages (furthermore a full AFL) and hence satisfy many closure properties.
Gerald Gazdar (1988)[3] and Vijay-Shanker (1987)[4] introduced a mildly context-sensitive language class now known as linear indexed grammars (LIG).
[5] Linear indexed grammars have additional restrictions relative to IG.
LIGs are weakly equivalent (generate the same language class) as tree adjoining grammars.