The Collaborative International Dictionary of English v.0.48:

Theorbo \The*or"bo\, n. [F. th['e]orbe, t['e]orbe, formerly
   tuorbe, tiorbe, It. tiorba.] (Mus.)
   An instrument made like large lute, but having two necks,
   with two sets of pegs, the lower set holding the strings
   governed by frets, while to the upper set were attached the
   long bass strings used as open notes.
   [1913 Webster]

   Note: A larger form of theorbo was also called the
         archlute, and was used chiefly, if not only, as an
         accompaniment to the voice. Both have long fallen into
         disuse.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Archlute \Arch"lute\, Archilute \Arch"i*lute\, n. [Cf. F.
   archiluth, It. arciliuto.] (Mus.)
   A large theorbo, or double-necked lute, formerly in use,
   having the bass strings doubled with an octave, and the
   higher strings with a unison.
   [1913 Webster]