A cour is a three-month block of television broadcasting, corresponding to one season of the year, and the standard unit anime series are scheduled in. One cour is usually 11 to 14 weekly episodes, most often 12 or 13. The word comes from the French cours via the Japanese kuuru.
Japanese television is programmed in quarters, and anime inherits the grid. The broadcast year divides into winter (January to March), spring (April to June), summer (July to September), and fall (October to December), and series are commissioned to fill one or more of those blocks. A weekly series filling one block airs about thirteen episodes, which is why so much anime comes in twelves and thirteens, and why a long show's episode count is often a multiple of a cour plus or minus a recap.
Cours compose in a few standard shapes. A single cour stands alone. A double cour runs two blocks back to back. A split cour airs its two blocks with a season or more of rest between them, a scheduling pattern that became common in the 2010s. Long-running franchises ignore the grid entirely and air for years.
The term is load-bearing vocabulary on this site, which is why the anime hub stamps a finished subject COURS COMPLETE. Knowing the cour is knowing the unit the industry itself thinks in.
In use"A 24-episode series airing April through September ran for two cours."