The AB language refers to a medieval literary dialect first identified by J.R.R. Tolkien in the article "Ancrene Wisse and Hali Meiðhad" (published in Essays and Studies by Members of the English Association, 14).
Tolkien had found that the Middle English spelling habits of the manuscript Ancrene Wisse (i.e., "A") showed great similarities to the manuscripts of the Katherine Group (i.e., "B"). This lead Tolkien to propose the existence of a standard literary language (AB language) in the West Midland area of England in the 13th century.
According to Arne Zettersten, Tolkien's research on the AB language was perhaps his most important academic contribution, influencing the work of his student and colleague Simonne d'Ardenne and Zettersten himself, as well as contemporary research on Middle English.