[s18e5] Episode #18.5 Here

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:

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. [S18E5] Episode #18.5

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