The belief that God can “concede” to common worldviews and practices for a time in order to make a point or advance his perfect will one step at a time.
For example: (although I don’t necessarily believe this) in the details of the creation narrative, God could have conceded to the common worldview about creation in describing the firmament and the days of creation in order to make the larger point that he is the ultimate creator.
Another example in practice is God’s tolerance of slavery in the Old Testament in order to advance his perfect plan slowly.
We all believe at least in a small form of concessionism in that we allow for phenomenological language (ie “the sun stood still” in Joshua).
We even practice this today quite a bit. An example may be when we draw an angel with wings as I did in my cover photo on this blog or we all do at Christmas time. We don’t really believe all angels have wings, but since this is the cultural way to identify angels, we concede in order to draw attention to something more important.
(I have been teaching about concessionism for so long I just took it for granted as being a technical term in theology. However, I recently did a search to see where I first heard of this and for the life of me cannot find this technical term. However, it is still valuable for my purpose and I will continue to use it until I find something better.)