The Collaborative International Dictionary of English v.0.48:

Collateral \Col*lat"er*al\ (k[o^]l*l[a^]t"[~e]r*al), a. [LL.
   collateralis; col- + lateralis lateral. See Lateral.]
   1. Coming from, being on, or directed toward, the side; as,
      collateral pressure. "Collateral light." --Shak.
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   2. Acting in an indirect way.
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            If by direct or by collateral hand
            They find us touched, we will our kingdom give . . .
            To you in satisfaction.               --Shak.
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   3. Related to, but not strictly a part of, the main thing or
      matter under consideration; hence, subordinate; not chief
      or principal; as, collateral interest; collateral issues.
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            That he [Attebury] was altogether in the wrong on
            the main question, and on all the collateral
            questions springing out of it, . . . is true.
                                                  --Macaulay.
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   4. Tending toward the same conclusion or result as something
      else; additional; as, collateral evidence.
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            Yet the attempt may give
            Collateral interest to this homely tale.
                                                  --Wordsworth.
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   5. (Genealogy) Descending from the same stock or ancestor,
      but not in the same line or branch or one from the other;
      -- opposed to lineal.
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   Note: Lineal descendants proceed one from another in a direct
         line; collateral relations spring from a common
         ancestor, but from different branches of that common
         stirps or stock. Thus the children of brothers are
         collateral relations, having different fathers, but a
         common grandfather. --Blackstone.
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   Collateral assurance, that which is made, over and above
      the deed itself.

   Collateral circulation (Med. & Physiol.), circulation
      established through indirect or subordinate branches when
      the supply through the main vessel is obstructed.

   Collateral issue. (Law)
      (a) An issue taken upon a matter aside from the merits of
          the case.
      (b) An issue raised by a criminal convict who pleads any
          matter allowed by law in bar of execution, as pardon,
          diversity of person, etc.
      (c) A point raised, on cross-examination, aside from the
          issue fixed by the pleadings, as to which the answer
          of the witness, when given, cannot subsequently be
          contradicted by the party asking the question.

   Collateral security, security for the performance of
      covenants, or the payment of money, besides the principal
      security.

   collateral damage, (Mil.) damage caused by a military
      operation, such as a bombing, to objects or persons not
      themselves the intended target of the attack.
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Bouvier's Law Dictionary, Revised 6th Ed (1856):

COLLATERAL SECURITY, contracts. A separate obligation attached to another
contract, to guaranty its performance. By this term is also meant the
transfer of property or of other contracts to insure the performance of a
principal engagement. The property or securities thus conveyed are also
called collateral securities. 1 Pow. Mortg. 393; 2 Id. 666, n. 871; 3 Id.
944, 1001.