Molinism (Middle Knowledge)

Molina, a Jesuit theologian in the post-Reformation period, argued that God has three logical moments of knowledge prior to creating the universe. God not only possesses knowledge of what could be, i.e. knowledge of all bare possibilities and logical necessities (what Molina calls “natural knowledge”), and knowledge of what will be, i.e. knowledge of all future actualities, or exact and detailed knowledge of the way the world, when created, will be (what Molina calls “free knowledge”), but importantly, God also possesses knowledge of what would be if circumstances were different from what they in fact will be in the actual world, i.e. knowledge of those possible states of affairs which would have become actual had circumstances other than those in the real world obtained (what Molina calls “middle knowledge”).
Bruce Ware from God's Greater Glory pg. 110

Scientia Media. Literally "middle knowledge." Many theologians have said that God knows the world by knowing himself. He knows what is possible or impossible in the world by knowing what he can or cannot do: this knowledge is called the knowledge of simple intelligence or necessary knowledge (since it follows from the very nature of God's being). He also knows what actually takes place in the world (whether past, present, or future from our point of view) by knowing his own plan, his decree for the world: this knowledge is called the knowledge of vision or free knowledge (since it follows from God's free decisions concerning the world process). Such a distinction was made by Thomas Aquinas and his Dominican followers. But in the sixteenth century the theologians of the new Jesuit order, particularly Luis Molina, seeking to restate the Roman Catholic theology in opposition to the challenges of Protestantism and Jansenism, found this distinction inadequate to do justice to human freedom. They introduced a third form of divine knowledge, a middle knowledge or scientia media. This knowledge (a) is a knowledge of what would happen under such-and-such conditions, and (b) is based, neither upon God's nature nor upon his decree, but upon the free decisions of created beings. Thus God knows what will happen if David re­mains in Keilah, and what will happen if he does not (I Sam. 23:1-13); and he knows it, not because he controls the course of history, but because he knows what free decisions people will make in­dependently of his controlling decree. This con­cept found favor with Lutherans (e.g., Quenstedt) and with Arminius and some of his followers. The Reformed agree that God knows what would happen under all conditions, but they reject the notion that this knowledge is ever ultimately based on man's autonomous decisions. Human decisions, they argue, are themselves the effects of God's eternal decrees (see Acts 2:23, Rom. 9:10-18, Eph. 1:11, Phil. 2:12-13).
John M. Frame "Scientia Media" from Evangelical Dictionary of Theology