In the general sense, propitiation is the act of appeasing an offended power; to reconcile, to attain friendship or goodwill, attempt to make compatible.
In a spiritual sense, God is the power whom we have offended by our wrong actions and behaviors. In and of ourselves, we cannot appease him or gain good standing with him, but Jesus has acted on our behalf to accomplish that as our propitiation.
A related word, that is more commonly used in the Bible, is atonement.
Key verse about propitiation: “And He Himself is the propitiation for our sins, and not for ours only but also for the whole world (Romans 3:25, NTL).
A lifelong student of the Bible, Peter DeHaan, PhD, wrote the 1,000-page website ABibleADay.com to encourage people to explore the Bible. His main blog and many books urge Christians to push past the status quo and reconsider how they practice their faith in every area of their lives.