The Collaborative International Dictionary of English v.0.48:

Subdeacon \Sub*dea"con\, n. [Pref. sub- + deacon: cf. L.
   subdiaconus.] (Eccl.)
   One belonging to an order in the Roman Catholic Church, next
   interior to the order of deacons; also, a member of a minor
   order in the Greek Church.
   [1913 Webster] Subdeaconry

WordNet (r) 3.0 (2006):

subdeacon
    n 1: a clergyman an order below deacon; one of the Holy Orders
         in the unreformed western Christian church and the eastern
         Catholic Churches but now suppressed in the Roman Catholic
         Church