The Collaborative International Dictionary of English v.0.48:

Rubaiyat \Ru*bai*yat"\, n. pl.; sing. Rubai. [Ar.
   rub[=a]'iy[=a]h quatrian, pl. of rub[=a]'iy having four
   radicals, fr. rub[=a]' four.]
   Quatrians; as, the Rubaiyat of Omar Khayyam. Sometimes in pl.
   construed as sing., a poem in such stanzas.
   [Webster 1913 Suppl.]