The Collaborative International Dictionary of English v.0.48:

Tumble \Tum"ble\ (t[u^]m"b'l), v. i. [imp. & p. p. Tumbled
   (t[u^]m"b'ld); p. pr. & vb. n. Tumbling (t[u^]m"bl[i^]ng).]
   [OE. tumblen, AS. tumbian to turn heels over head, to dance
   violently; akin to D. tuimelen to fall, Sw. tumla, Dan.
   tumle, Icel. tumba; and cf. G. taumeln to reel, to stagger.]
   1. To roll over, or to and fro; to throw one's self about;
      as, a person in pain tumbles and tosses.
      [1913 Webster]

   2. To roll down; to fall suddenly and violently; to be
      precipitated; as, to tumble from a scaffold.
      [1913 Webster]

            He who tumbles from a tower surely has a greater
            blow than he who slides from a molehill. --South.
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   3. To play tricks by various movements and contortions of the
      body; to perform the feats of an acrobat. --Rowe.
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   To tumble home (Naut.), to incline inward, as the sides of
      a vessel, above the bends or extreme breadth; -- used esp.
      in the phrase tumbling home. Cf. Wall-sided.
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The Collaborative International Dictionary of English v.0.48:

Tumbling \Tum"bling\,
   a. & vb. n. from Tumble, v.
   [1913 Webster]

   Tumbling barrel. Same as Rumble, n., 4.

   Tumbling bay, an overfall, or weir, in a canal.
      [1913 Webster] Tumbrel

WordNet (r) 3.0 (2006):

tumbling
    n 1: the gymnastic moves of an acrobat [syn: acrobatics,
         tumbling]