The Collaborative International Dictionary of English v.0.48:

Lawful \Law"ful\, a.
   1. Conformable to law; allowed by law; legitimate; competent.
      [1913 Webster]

   2. Constituted or authorized by law; rightful; as, the lawful
      owner of lands.
      [1913 Webster]

   Lawful age, the age when the law recognizes one's right of
      independent action; majority; -- generally the age of
      twenty-one years. Also called legal age or age of
      majority.
      [1913 Webster +PJC]

   Note: In some of the States, and for some purposes, a woman
         attains lawful age at eighteen. --Abbott.

   Syn: Legal; constitutional; allowable; regular; rightful.

   Usage: Lawful, Legal. Lawful means conformable to the
          principle, spirit, or essence of the law, and is
          applicable to moral as well as juridical law. Legal
          means conformable to the letter or rules of the law as
          it is administered in the courts; conformable to
          juridical law. Legal is often used as antithetical to
          equitable, but lawful is seldom used in that sense. --
          Law"ful*ly, adv. -- Law"ful*ness, n.
          [1913 Webster]

WordNet (r) 3.0 (2006):

lawful
    adj 1: conformable to or allowed by law; "lawful methods of
           dissent" [ant: unlawful]
    2: according to custom or rule or natural law [syn: lawful,
       rule-governed]
    3: having a legally established claim; "the legitimate heir";
       "the true and lawful king" [syn: true(a), lawful,
       rightful(a)]
    4: authorized, sanctioned by, or in accordance with law; "a
       legitimate government" [syn: lawful, legitimate, licit]

Moby Thesaurus II by Grady Ward, 1.0:

125 Moby Thesaurus words for "lawful":
   actionable, admissible, allowable, allowed, applicable, authentic,
   authoritative, authorized, balanced, binding, bona fide, candid,
   card-carrying, cogent, competent, consistent, constitutional,
   de jure, defensible, deserved, dinkum, due, equitable, even,
   evenhanded, fair, fair and square, fit, following the letter,
   genuine, good, honest, honest-to-God, inartificial, judicial,
   juridical, just, justiciable, justifiable, justified, kosher,
   lawmaking, legal, legislative, legit, legitimate, legitimized,
   level, licit, lifelike, literal, logical, meet, meet and right,
   merited, natural, naturalistic, original, permissible, permitted,
   proper, pure, real, realistic, right, right and proper, rightful,
   sanctionable, sanctioned, self-consistent, simon-pure, simple,
   sincere, solid, sound, square, statutory, sterling, substantial,
   sufficient, sure-enough, true to life, true to nature,
   true to reality, unadulterated, unaffected, unassumed, unassuming,
   uncolored, unconcocted, uncopied, uncounterfeited, undisguised,
   undisguising, undistorted, unexaggerated, unfabricated, unfanciful,
   unfeigned, unfeigning, unfictitious, unflattering, unimagined,
   unimitated, uninvented, unpretended, unpretending, unqualified,
   unromantic, unsimulated, unspecious, unsynthetic, unvarnished,
   valid, verbal, verbatim, veridical, verisimilar, warrantable,
   warranted, weighty, well-founded, well-grounded, within the law,
   word-for-word



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

LAWFUL. That which is not forbidden by law. Id omne licitum est, quod non
est legibus prohibitum, quamobrem, quod, lege permittente, fit, poenam non
meretur. To be valid a contract must be lawful.



The Devil's Dictionary (1881-1906):

LAWFUL, adj.  Compatible with the will of a judge having jurisdiction.