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.
      [1913 Webster]

   2. Acting in an indirect way.
      [1913 Webster]

            If by direct or by collateral hand
            They find us touched, we will our kingdom give . . .
            To you in satisfaction.               --Shak.
      [1913 Webster]

   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.
      [1913 Webster]

            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.
      [1913 Webster]

   4. Tending toward the same conclusion or result as something
      else; additional; as, collateral evidence.
      [1913 Webster]

            Yet the attempt may give
            Collateral interest to this homely tale.
                                                  --Wordsworth.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
         [1913 Webster]

   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.
      [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Collateral \Col*lat"er*al\, n.
   1. A collateral relative. --Ayliffe.
      [1913 Webster]

   2. Collateral security; that which is pledged or deposited as
      collateral security.
      [1913 Webster]

WordNet (r) 3.0 (2006):

collateral
    adj 1: descended from a common ancestor but through different
           lines; "cousins are collateral relatives"; "an indirect
           descendant of the Stuarts" [syn: collateral,
           indirect] [ant: direct, lineal]
    2: serving to support or corroborate; "collateral evidence"
       [syn: collateral, confirmative, confirming,
       confirmatory, corroborative, corroboratory,
       substantiating, substantiative, validating,
       validatory, verificatory, verifying]
    3: accompany, concomitant; "collateral target damage from a
       bombing run"
    4: situated or running side by side; "collateral ridges of
       mountains"
    n 1: a security pledged for the repayment of a loan

Moby Thesaurus II by Grady Ward, 1.0:

209 Moby Thesaurus words for "collateral":
   accessory, accident, accidental, accompanying, addendum, addition,
   additional, adjunct, adscititious, adventitious, affiliate,
   affiliated, agnate, agreeing, akin, aligned, allied, an, analogous,
   ancestry, ancillary, another, appendage, appurtenance, appurtenant,
   ascititious, associate, associated, attendant, attending,
   auxiliary, avuncular, blood, blood relation, blood relative, bound,
   bracketed, casual, caution, caution money, circumstantial,
   clansman, closely related, coetaneous, coeternal, coeval,
   coexistent, coexisting, coextending, coextensive, cognate,
   coincident, coinstantaneous, collateral relative,
   collateral security, combined, coming, concomitant, concurrent,
   congeneric, conjoint, conjugate, connected, connections,
   consanguine, consanguinean, consanguineous, contemporaneous,
   contemporary, conterminous, contingency, contingent, contributory,
   correlated, correlative, coterminous, coupled, debris, deposit,
   distaff side, distant relation, distantly related, enate, equal,
   equidistant, equispaced, even, eventual, extra, family, farther,
   fellow, final, flesh, flesh and blood, folks, forfeit, fortuitous,
   foster, fresh, further, garbage, german, germane, happenstance,
   implicated, incidental, indirect, inessential, interlinked,
   interlocked, interrelated, involved, isochronal, isochronous,
   joined, joint, junk, kelter, kin, kindred, kinfolk, kinnery,
   kinsfolk, kinsman, kinsmen, kinswoman, kith and kin, knotted, last,
   lined up, linked, litter, margin, matrilateral, matrilineal,
   matroclinous, mere chance, more, mutual, near relation, new,
   next of kin, nonconvergent, nondivergent, nonessential, not-self,
   novercal, of that ilk, of that kind, of the blood, offal, other,
   paired, parallel, parallelepipedal, parallelinervate, paralleling,
   parallelodrome, parallelogrammatic, parallelogrammic,
   parallelotropic, patrilateral, patrilineal, patroclinous, people,
   plus, posterity, related, relations, relatives, riffraff, rubbish,
   secondary, sib, sibling, simultaneous, spare, spear kin,
   spear side, spindle kin, spindle side, spliced, stake, subsidiary,
   superadded, superaddition, superfluous, supernumerary,
   supervenient, supplement, supplemental, supplementary, surplus,
   sword side, tied, tribesman, twin, twinned, ulterior, ultimate,
   unessential, unison, unisonous, uterine, uterine kin, wed, wedded,
   yoked



Bouvier's Law Dictionary, Revised 6th Ed (1856):

COLLATERAL, collateralis. From latus, a side; that which is sideways, and
not direct.