If you are looking for a show specifically using the notation (often used for mid-season recaps or specials), it likely refers to one of the following:
In Season 18 (projected/upcoming), Episode 5 involves a multi-week challenge where contestants must form alliances and rank each other. [S18E5] Episode #18.5
Jeremy Clarkson tests the Maserati GranTurismo MC Stradale , evaluating if it's too hardcore for road use. Other Possible "Episode 18.5" Matches If you are looking for a show specifically