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.
An unconstitutional mode of obtaining aid. --Hallam.
2. The person or thing that promotes or helps in something
done; a helper; an assistant.
It is not good that man should be alone; let us make
unto him an aid like unto himself. --Tobit viii.
3. (Eng. Hist.) A subsidy granted to the king by Parliament;
also, an exchequer loan.
4. (Feudal Law) A pecuniary tribute paid by a vassal to his
lord on special occasions. --Blackstone.
5. An aid-de-camp, so called by abbreviation; as, a general's
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.
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.