The Collaborative International Dictionary of English v.0.48:

Testudo \Tes*tu"do\, n.; pl. Testudines. [L., from testa the
   shell of shellfish, or of testaceous animals.]
   1. (Zool.) A genus of tortoises which formerly included a
      large number of diverse forms, but is now restricted to
      certain terrestrial species, such as the European land
      tortoise (Testudo Graeca) and the gopher of the Southern
      United States.
      [1913 Webster]

   2. (Rom. Antiq.) A cover or screen which a body of troops
      formed with their shields or targets, by holding them over
      their heads when standing close to each other. This cover
      resembled the back of a tortoise, and served to shelter
      the men from darts, stones, and other missiles. A similar
      defense was sometimes formed of boards, and moved on
      wheels.
      [1913 Webster]

   3. (Mus.) A kind of musical instrument. a species of lyre; --
      so called in allusion to the lyre of Mercury, fabled to
      have been made of the shell of a tortoise.
      [1913 Webster]

WordNet (r) 3.0 (2006):

testudo
    n 1: a movable protective covering that provided protection from
         above; used by Roman troops when approaching the walls of a
         besieged fortification
    2: type genus of the Testudinidae [syn: Testudo, genus
       Testudo]