In modern musical parlance, a hemiola is a metrical pattern in which two bars in triple time (3/2 or 3/4 for example) are articulated as if they were three bars in duple time (2/2 or 2/4).
The word hemiola derives from the Greek hemiolios, meaning "one and a half". (The term hemiola or "one and a half" was also used by the Greeks to refer to a galley powered by one and a half banks of oars). It was originally used in music to refer to the frequency ratio 3:2; that is, the interval of a justly tuned perfect fifth.
Later, from around the 15th century, the word came to mean the use of three breves in a bar when the prevailing metrical scheme had two dotted breves in each bar. This usage was later extended to its modern sense of two bars in triple time articulated or phrased as if they were three bars in duple time.
Currently listening to: "Bloodletting upon the Cloven Hoof" by Goatwhore (surely a contender for the most autofarcical song name EVAR)