What is "augmented sixth chord"

n. (context music English) a chord that contains a lowered submediant scale degree and a raised subdominant scale degree

In music theory, an augmented sixth chord contains the interval of an augmented sixth, usually above its bass tone. This chord has its origins in the Renaissance, further developed in the Baroque, and became a distinctive part of the musical style of the Classical and Romantic periods. Conventionally used with a predominant function ( resolving to the dominant), the three more common types of augmented sixth chords are usually called Italian sixth, French sixth, and German sixth.