An apologist was an individual who defended another's position, particularly a contentious one.
In 2374, Vedek Yassim accused Kira Nerys of becoming an apologist for the Dominion, whose actions he viewed as evil. He was frustrated at her position and questioned what would have to happen for her to cease accepting them and begin to rebel. (DS9: "Rocks and Shoals")
In 2381, Nick Locarno accused Beckett Mariner (who he had previously believed to be of the same mind as him) of being "just another apologist for a broken system!'" (namely, Starfleet) (LD: "Old Friends, New Planets")