Monergism (Greek mono meaning "one" and erg meaning "work") is a term for the belief that the Holy Spirit is the only agent who effects regeneration of Christians. This view, held by Reformed and Calvinistic groups, sees salvation as the work of God alone, from first to last. He has chosen in eternity past whom He will save out of lost humanity (often referred to as the elect), and in His timing He will bring the elect to faith through the work of the Spirit for the sake of the Son, and save them forever to the praise of His glorious grace (Romans 8:29f). This is opposed to the synergistic view as held by Arminianism and its theological predecessor Semi-Pelagianism where salvation is seen as a cooperative effort between God and man.
Quoting John Hendryx, "Monergism simply means that it is God who gives ears to hear and eyes to see. It is God alone who gives illumination and understanding of His word that we might believe; It is God who raises us from the dead, who circumcises the heart; unplugs our ears; It is God alone who can give us a new sense that we may, at last, have the moral capacity to behold His beauty and unsurpassed excellency." - A Simple Explanation of Monergism