The Collaborative International Dictionary of English v.0.48:

Aid \Aid\, n. [F. aide, OF. a["i]de, a["i]e, fr. the verb. See
   Aid, v. t.]
   1. Help; succor; assistance; relief.
      [1913 Webster]

            An unconstitutional mode of obtaining aid. --Hallam.
      [1913 Webster]

   2. The person or thing that promotes or helps in something
      done; a helper; an assistant.
      [1913 Webster]

            It is not good that man should be alone; let us make
            unto him an aid like unto himself.    --Tobit viii.
                                                  6.
      [1913 Webster]

   3. (Eng. Hist.) A subsidy granted to the king by Parliament;
      also, an exchequer loan.
      [1913 Webster]

   4. (Feudal Law) A pecuniary tribute paid by a vassal to his
      lord on special occasions. --Blackstone.
      [1913 Webster]

   5. An aid-de-camp, so called by abbreviation; as, a general's
      aid.
      [1913 Webster]

   Aid prayer (Law), a proceeding by which a defendant
      beseeches and claims assistance from some one who has a
      further or more permanent interest in the matter in suit.


   To pray in aid, to beseech and claim such assistance.
      [1913 Webster]

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

AID PRAYER, English law. A petition to the court calling in help from
another person who has an interest in the matter in dispute. For example, a
tenant for life, by the courtesy or for years, being impleaded, may pray aid
of him in reversion; that is, desire the court that he may be called by
writ, to allege what he thinks proper for the maintenance of the right of
the person calling him, and of his own. F. N. B. 60; Cowel.