Speaking in tongues, or glossolalia, is the practice, central to Pentecostal and Charismatic Christianity, of vocalising fluent speech-like sounds that the speaker does not understand and believes to be prompted by the Holy Spirit. It is understood by practitioners as a spiritual gift described in the New Testament: the Book of Acts records the disciples at Pentecost speaking in other tongues as the Spirit gave them utterance, and the Apostle Paul discusses the gift at length among the charisms given to the church, treating it as a form of prayer and praise addressed to God that edifies the speaker, and directing that in public worship it be exercised in order and accompanied by interpretation. For Pentecostals the experience is often linked to the "baptism in the Holy Spirit," a distinct empowering subsequent to conversion, of which tongues may be the sign.
The practice, largely dormant in mainstream Christianity for centuries, re-emerged with great force in the Pentecostal revival at the start of the twentieth century and spread through the Charismatic movement into many denominations, becoming one of the most recognisable features of a now vast global Christian current. It has been studied by linguists and psychologists, who generally find glossolalia to be not a natural human language but a patterned vocalisation, and who have examined it as a phenomenon of religious experience without settling the theological question of its source. Within Christianity it remains variously esteemed and disputed, prized by Pentecostals and Charismatics as a living gift of the Spirit, regarded by others as having ceased with the apostolic age or as requiring careful discernment, and it marks one of the clearest dividing lines in contemporary Protestant practice.