The Collaborative International Dictionary of English v.0.48:

Spondee \Spon"dee\ (sp[o^]n"d[=e]), 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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

spondee
    n 1: a metrical unit with stressed-stressed syllables