A hemiola: three half notes that span over the time of two full bars. This could also be thought of as 3:2 (three over two), an example of a polyrhythm.
A hemiola is the imposition of a pattern of rhythm or articulation other than that implied by the time signature; specifically, in triple time (for example in 3/4) the imposition of a duple pattern (as if the time signature were, for example, 2/4). A useful phrase by which to think of a hemiola is "over the bar". A hemiola is sometimes placed at the end of a piece to act as a kind of rallentando. See also syncopation.