NOUN (2)

1. as the agent of or on someone's part (usually expressed as "on behalf of"; rather than "in behalf of");
- Example: "the guardian signed the contract on behalf of the minor child"
- Example: "this letter is written on behalf of my client";

2. for someone's benefit (usually expressed as `in behalf' rather than `on behalf' and usually with a possessive);
- Example: "in your behalf"
- Example: "campaigning in his own behalf"
- Example: "spoke a good word in his friend's behalf"

The Collaborative International Dictionary of English v.0.48:

Behalf \Be*half"\, n. [OE. on-behalve in the name of, bihalven
   by the side of, fr. AS. healf half, also side, part: akin to
   G. halb half, halber on account of. See Be-, and Half,
   n.]
   Advantage; favor; stead; benefit; interest; profit; support;
   defense; vindication.
   [1913 Webster]

         In behalf of his mistress's beauty.      --Sir P.
                                                  Sidney.
   [1913 Webster]

         Against whom he had contracted some prejudice in behalf
         of his nation.                           --Clarendon.
   [1913 Webster]

   In behalf of, in the interest of.

   On behalf of, on account of; on the part of.
      [1913 Webster]

WordNet (r) 3.0 (2006):

behalf
    n 1: as the agent of or on someone's part (usually expressed as
         "on behalf of" rather than "in behalf of"); "the guardian
         signed the contract on behalf of the minor child"; "this
         letter is written on behalf of my client";
    2: for someone's benefit (usually expressed as `in behalf'
       rather than `on behalf' and usually with a possessive); "in
       your behalf"; "campaigning in his own behalf"; "spoke a good
       word in his friend's behalf"

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "behalf":
   advantage, avail, behoof, benefit, benison, blessing, boon,
   convenience, for, gain, good, in place of, interest, percentage,
   point, profit, service, use, value, welfare, well-being,
   world of good, worth