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.
Jeremy Clarkson tests the Maserati GranTurismo MC Stradale , evaluating if it's too hardcore for road use. Other Possible "Episode 18.5" Matches [S18E5] Episode #18.5
Episode 18.5, titled "All of This," is a recap episode that summarizes the events of the series up to that point. Community consensus on Reddit generally suggests it is skippable if you are binge-watching.
Episode 18.5, "Recollection," is similarly a compilation episode summarizing the first half of the Alicization arc. If you are looking for a show specifically
Jeremy Clarkson and James May explore the history of Saab, highlighting its engineering eccentricities, such as the ignition being located between the seats and the company's roots in aviation.
Matt Smith, then the lead in Doctor Who , joins the show to set a lap time on the Top Gear track. Community consensus on Reddit generally suggests it is
Season 18, Episode 5, titled "The Serpent's Cove," follows a veteran and a cattle rancher trying to survive 21 days in the Philippine jungle.