The efficacy of prayer as a petition to a deity is usually evaluated with regard to the concept of prayer healing. There have been numerous studies done, with contradictory results. There has been some criticism of the way the studies were conducted.

Etymology

Pray entered Middle English as preyen, prayen,and preien around 1290, recorded in The early South-English Legendary I. 112/200: And preide is fader wel latinx: ȝerne, in the sense of "to ask earnestly." The next recorded use in 1300 is simply "to pray." The word came to English from Old French preier, "to request" (first seen in La Séquence de Ste. Eulalie, ca. 880) In modern French prier, "to pray," the stem-vowel is leveled under that of the stem-stressed forms, il prie, etc. The origin of the word before this time is less certain. Compare the Italian Pregare, "to ask" or more rarely "pray for something" and Spanish preguntar, "ask."

One possibility is the Late Latin precare (as seen in Priscian), classical Latin precari "to entreat, pray" from Latin precari, from precor, from prec-, prex "request, entreaty, prayer." Precor was used by Virgil, Livy, Cicero, and Ovid in the accusative. Dative forms are also found in Livy and Aurelius Propertius. With pro in the ablative, it is found in Plinius Valerianus's physic, and Aurelius Augustinus'sEpistulae. It also could be used for a thing. From classical times, it was used in both religious and secular senses. Prex is recorded as far back as T. Maccius Plautus (254 B.C. – ?). Other senses of precor include "to wish well or ill to any one," "to hail, salute," or "address one with a wish."

The Latin orare "to speak" later took over the role of precari to mean "pray." The Middle English word Orison, whose meaning in modern English has been taken over by Prayer, has been derived from this word via the Old French word oraison.

The Spanish form preguntar was first recorded in El Cantar de Mio Çid (ca. 1150) and possibly comes from Vulgar Latin praecontare, an alteration of the Classical Latin percontari, perconto, percontor "interrogate" although the Spanish verb for "pray" today is (among Catholics) rezar, which previously meant "to say" from the Latin recitare. Among Spanish-speaking Protestants, the verb orar is used instead, and a prayer is called oración. The Portuguese word pregar "to preach," or less commonly, "to exhort," is also mentioned at times, although it is from the Latin praedicare,"to cry in public, proclaim," hence "to declare, state, say," in medieval Latin "to preach," and in Logic "to assert," from præ "forth" + dicare "to make known, proclaim." Compare the Spanish predicar. More closely related is the Portuguese perguntar, "to ask" and by extension "ask for." Pray is akin to Old English gefr1ge "hearsay, report," fricgan, frignan, frinan to ask, inquire, Old High German fraga question, fragen "to ask" (in modern German, "pray" is beten, "question" frage), Old Norse frett "question," fregna "to inquire, find out," Gothic fraihman "to find out by inquiry," Tocharian A prak- "to ask," Sanskrit roots, pracch- prask-, pras "interrogation," and prcchati "he asks"