The Collaborative International Dictionary of English v.0.48:

Sputum \Spu"tum\ (sp[=u]"t[u^]m), n.; pl. Sputa (-t[.a]). [L.,
   from spuere, sputum, to spit.]
   That which is expectorated; a salival discharge; spittle;
   saliva.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sputum
    n 1: expectorated matter; saliva mixed with discharges from the
         respiratory passages; in ancient and medieval physiology it
         was believed to cause sluggishness [syn: phlegm,
         sputum]