A "guest preacher" is someone who is invited to give a sermon or speak at a religious service, but is not the regular or resident preacher of that place. Full definition
We've had guest preachers during Lenten services who asked if we would like to follow along in the Bible, and then they say «the verse is such and such.» (religion-online.org)
I recall one of my colleagues in our liberal seminary storming out of chapel because the guest preacher used the term «Kingdom» too many times instead of «kindom,» «realm of God,» «ground of our being» or some other more progressive / liberal designation. (nakedpastor.com)