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:
This episode, which originally aired on February 26, 2012, is best known for its heartfelt and quirky tribute to the Swedish car manufacturer , following its bankruptcy.
The code typically refers to the fifth episode of an eighteenth season. The most prominent show with this specific numbering is the legendary motoring program Top Gear . Top Gear (UK) - Series 18, Episode 5 [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
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
Richard Hammond conducts a spectacular race in the Alps, pitting a rally-spec Skoda Fabia S2000 against Swiss stuntman Yves Rossy, who uses a rocket-powered wing to fly through the mountains.
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. Top Gear (UK) - Series 18, Episode 5
In Season 18 (projected/upcoming), Episode 5 involves a multi-week challenge where contestants must form alliances and rank each other.