A pump..

and while we are on the subject--
what do you call the high heeled shoes so popular today, they are almost slippers, with open toes, and a strap or two over the instep, and open backed..but definately not slippers, with a heel from 1 to almost 3 inches high.

(i really have a problem seeing these shoes being worn to work.. but i seem to be alone in that opinion.)
i would call them a mule