Here's part of what Grove Music gives as a definition:

'Hemiola' in the modern metrical system denotes the articulation of two units of triple metre as if they were notated as three units of duple metre. This is a common feature of Baroque music, especially of the French courante, and is used for giving rhythmic variety to dances and helping to effect an allargando at the end of a longer movement; Handel made much use of it. In the 19th century it was used by Schumann and often by Brahms, and was an important feature of the Viennese waltz. Hemiola is a distinguishing feature of such folkdances as the Andalusian polo and the Central American huapango, rhythmic characteristics of which were incorporated by Bernstein into 'America' from West Side Story