The Collaborative International Dictionary of English v.0.48:

Legal \Le"gal\ (l[=e]"gal), a. [L. legalis, fr. lex, legis, law;
   prob. orig., that which lies or is fixed (cf. L. lectus bed),
   and if so akin to E. lie, law: cf. F. l['e]gal. Cf. Lie to
   be prostrate, Loyal, Leal.]
   1. Created by, permitted by, in conformity with, or relating
      to, law; as, a legal obligation; a legal standard or test;
      a legal procedure; a legal claim; a legal trade; anything
      is legal which the laws do not forbid.
      [1913 Webster]

   2. (Theol.)
      (a) According to the law of works, as distinguished from
          free grace; or resting on works for salvation.
      (b) According to the old or Mosaic dispensation; in
          accordance with the law of Moses.
          [1913 Webster]

   3. (Law) Governed by the rules of law as distinguished from
      the rules of equity; as, legal estate; legal assets.
      --Bouvier. --Burrill.
      [1913 Webster]

   Legal cap. See under Cap.

   Legal tender.
      (a) The act of tendering in the performance of a contract
          or satisfaction of a claim that which the law
          prescribes or permits, and at such time and place as
          the law prescribes or permits.
      (b) That currency, or money, which the law authorizes a
          debtor to tender and requires a creditor to receive.
          It differs in different countries.

   Syn: Lawful; constitutional; legitimate; licit; authorized.
        See Lawful.
        [1913 Webster]

WordNet (r) 3.0 (2006):

legal
    adj 1: established by or founded upon law or official or
           accepted rules [ant: illegal]
    2: of or relating to jurisprudence; "legal loophole"
    3: having legal efficacy or force; "a sound title to the
       property" [syn: legal, sound, effectual]
    4: relating to or characteristic of the profession of law; "the
       legal profession"
    5: allowed by official rules; "a legal pass receiver"

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "legal":
   acceptable, actionable, admissible, allowable, applicable,
   authoritative, authorized, balanced, binding, booked, cogent,
   competent, consistent, constitutional, defensible, deserved,
   documented, down, due, enrolled, entered, equitable, even,
   evenhanded, fair, fair and square, filed, fit, forensic, good,
   indexed, inscribed, judicial, judiciary, juridical, just,
   justiciable, justifiable, justified, kosher, lawful, lawmaking,
   legislative, legit, legitimate, legitimized, level, licit, logged,
   logical, meet, meet and right, merited, minuted, of record,
   official, on file, on record, on the books, permissible, permitted,
   posted, proper, recorded, registered, right, right and proper,
   rightful, sanctionable, sanctioned, self-consistent, solid, sound,
   square, statutory, substantial, sufficient, valid, warrantable,
   warranted, weighty, well-founded, well-grounded, within the law,
   written down



The Jargon File (version 4.4.7, 29 Dec 2003):

legal
 adj.

    Loosely used to mean ?in accordance with all the relevant rules?, esp. in
    connection with some set of constraints defined by software. ?The older =+
    alternate for += is no longer legal syntax in ANSI C.? ?This parser
    processes each line of legal input the moment it sees the trailing
    linefeed.? Hackers often model their work as a sort of game played with the
    environment in which the objective is to maneuver through the thicket of
    ?natural laws? to achieve a desired objective. Their use of legal is
    flavored as much by this game-playing sense as by the more conventional one
    having to do with courts and lawyers. Compare language lawyer, legalese
    .


The Free On-line Dictionary of Computing (30 December 2018):

legal

   Loosely used to mean "in accordance with all the relevant
   rules", especially in connection with some set of constraints
   defined by software.  "The older =+ alternate for += is no
   longer legal syntax in ANSI C."  "This parser processes each
   line of legal input the moment it sees the trailing linefeed."
   Hackers often model their work as a sort of game played with
   the environment in which the objective is to maneuver through
   the thicket of "natural laws" to achieve a desired objective.
   Their use of "legal" is flavoured as much by this game-playing
   sense as by the more conventional one having to do with courts
   and lawyers.  Compare language lawyer, legalese.

   [Jargon File]


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

LEGAL. That which is according to law. It is used in opposition to
equitable, as the legal estate is, in the trustee, the equitable estate in
the cestui que trust. Vide Powell on Mortg. Index, h.t.
     2. The party who has the legal title, has alone the right to seek a
remedy for a wrong to his estate, in a court of law, though he may have no
beneficial interest in it. The equitable owner, is he who has not the legal
estate, but is entitled to the beneficial interest.
     3. The person who holds the legal estate for the benefit of another, is
called a trustee; he who has the beneficiary interest and does not hold the
legal title, is called the beneficiary, or more technically, the cestui que
trust.
     4. When the trustee has a claim, he must enforce his right in a court
of equity, for he cannot sue any one at law, in his own name; 1 East, 497; 8
T. R. 332; 1 Saund. 158, n. 1; 2 Bing. 20; still less can he in such court
sue his own trustee. 1 East, 497.