In Judaism, the Holy Spirit, otherwise known as the Holy Ghost, is the divine force, quality and influence of God over the universe or his creatures. In Nicene Christianity, the Holy Spirit is the third person of the Trinity. In Islam, the Holy Spirit acts as an agent of divine action or communication. In the Baha’i Faith, the Holy Spirit is seen as the intermediary between God and man and "the outpouring grace of God and the effulgent rays that emanate from His Manifestation".

For the majority of Christian denominations, the Holy Spirit, or Holy Ghost, is believed to be the third Person of the Trinity, a triune God manifested as God the Father, God the Son, and God the Holy Spirit, each being God. Nontrinitarian Christians, who reject the doctrine of the Trinity, differ significantly from mainstream Christianity in their beliefs about the Holy Spirit. In Christian theology, pneumatology is the study of the Holy Spirit. Due to Christianity's historical relationship with Judaism, theologians often identify the Holy Spirit with the concept of the Ruach Hakodesh in Jewish scripture, on the theory that Jesus was expanding upon these Jewish concepts. Similar names, and ideas, include the Ruach Elohim, Ruach YHWH, and the Ruach Hakodesh. In the New Testament the Holy Spirit is identified with the Spirit of Christ, the Spirit of Truth, and the Paraclete (helper).