Prayer is an invocation or act that seeks to activate a rapport with a deity, an object of worship, or a spiritual entity through deliberate communication. Prayer can be a form of religious practice, may be either individual or communal and take place in public or in private. It may involve the use of words or song. When language is used, prayer may take the form of a hymn, incantation, formal creed, or a spontaneous utterance in the praying person. There are different forms of prayer such as petitionary prayer, prayers of supplication, thanksgiving, and worship/praise. Prayer may be directed towards a deity, spirit, deceased person, or lofty idea, for the purpose of worshipping, requesting guidance, requesting assistance, confessing sins or to express one's thoughts and emotions. Thus, people pray for many reasons such as personal benefit or for the sake of others. Yoga is also a common form of prayer.

Most major religions involve prayer in one way or another. Some ritualize the act of prayer, requiring a strict sequence of actions or placing a restriction on who is permitted to pray, while others teach that prayer may be practiced spontaneously by anyone at any time.

Scientific studies regarding the use of prayer have mostly concentrated on its effect on the healing of sick or injured people. Meta-studies of the studies in this field have been performed showing evidence only for no effect or a potentially small effect. For instance, a 2006 meta analysis on 14 studies concluded that there is "no discernable effect" while a 2007 systemic review of intercessory prayer reported inconclusive results, noting that 7 of 17 studies had "small, but significant, effect sizes" but the review noted that the most methodologically rigorous studies failed to produce significant findings. Some studies have indicated increased medical complications in groups receiving prayer over those without. The efficacy of petition in prayer for physical healing to a deity has been evaluated in numerous other studies, with contradictory results. There has been some criticism of the way the studies were conducted.