Monergism
Monergism comes from two Greek words, μόνο (mono) meaning only and ἔργον (ergon) meaning a work or worker who accomplishes something. Together they mean “to work alone”. To summarize, it means God does the work to save us. In contrast we have Synergism, which means “to work together” and means that both God and man must work together for salvation.
GotQuestions.org explains the difference in this way…
The essence of the monergistic argument is that God is in the business of actually saving people and not merely making them “savable.” Monergism starts with an enemy of God, seemingly unsavable, and, by the grace of God, brings that spiritually dead person into saving faith and union with Christ. Synergism, in all its forms (including Pelagianism), starts with a person who has at least a spark of spiritual life. This person has the natural ability to take a step toward God apart from grace and thus meet God in the middle. God may do most of the saving work, but He must somehow also depend on the work of the individual being saved.
Traditionally Calvinists have been considered monergists and Arminians, synergists. The distinction is made a lot more clear by this excerpt from the site EvangelicalArminians.org…
After being enabled by the Spirit, the response of the sinner is passive. The sinner must stop resisting, repent of their sins, and place their faith in Christ. This gift, like any gift, is not irresistible. The sinner must accept the unmerited gift of God. Once this is done, following the plan of the Father, the Spirit joins the sinner to Jesus and thus begins the Savior’s relationship with the sinner.
This is the part of Arminianism one could call synergistic, the acceptance of the gift of salvation… The process of salvation is monergistic. He enables, He convicts, He draws, and He calls. Once the sinner places their faith in God, He is the one who justifies, sanctifies, and glorifies the sinner; just as He had predestined to do (Romans 8:29-30) because the work of Christ on the cross was made for our atonement.
This would fall under the umbrella of Semi-Pelagianism, in that it requires action on the part of man to complete salvation, and therefore is—in spite of the semantics by EvangelicArminians—synergism.