The Arminian belief that Jesus' sacrifice on the cross was for the entire world making all of mankind savable.  The atonement is effective only for those who believe but is available to all.

While this sounds like the most basic understanding of John 3:16 as well as various other scripture passages, limited atonement contends that "the world" does not mean every person in the world.

Unlimited atonement is in direct opposition to the Calvinist point of limited atonement and is supported by more than a few who are otherwise Calvinistic in their beliefs.  Calvinists who hold to this view but the other four points of Calvinism are known as four point Calvinists.