A type of metric foot
It consists of short, long and short syllables or morae, in that order. It is very hard to use in English; it works better in Latin or Greek.
One place it is heard in English is in the Guy Fawkes Day poem, which begins "Remember, remember, the fifth of November..."
Thanks to http://www.cs.rice.edu/~ssiyer/minstrels/poems/549.html