Source : Webster's Revised Unabridged Dictionary (1913)

Spondee \Spon"dee\, n. [L. spondeus, Gr. ? (sc. ?), fr. ? a
   drink offering, libation, fr. ? to pour out, make a libation:
   cf. F. spond['e]e. So called because at libations slow,
   solemn melodies were used, chiefly in this meter.] (pros.)
   A poetic foot of two long syllables, as in the Latin word
   l[=e]g[=e]s.

Source : WordNet®

spondee
     n : a metrical unit with stressed-stressed syllables