to decide on something in advance and hold to it; to purpose, plan, or intend
This is the settled, inward side of the word. Built from a base verb meaning "to place, set" with a prefix meaning "before," it pictures a person setting a plan out in front of their own mind and resolving on it. So it means to purpose or intend something ahead of time.
Paul uses it this way for himself when he says he had often planned to visit the believers in Rome. He also uses it of God: what God "purposed" points not to a sudden idea but to a design he formed long in advance. In the letter to the Ephesians this word sits in a run of terms about God's will and settled plan, and they reinforce each other.
When the subject is God, the word carries real theological weight, but the plain meaning stays the same: to decide on a course of action beforehand and keep to it.