Midseason replacement

In American and Canadian television, a mid-season replacement is a television series that premieres in the second half of the traditional television season, usually between January and May. Mid-season replacements usually take place after a show that was in the fall schedule was canceled or put on hiatus, outside factors such as an actor's family emergency or personal illness led to a delay in the program's debut, a program was purposefully scheduled for mid-season, or a program had a shortened season for some other reason which resulted in a time slot that needed filling. A few shows in American television history have been perennial mid-season replacements; American Idol, for example, from its second season onward, aired from January to May each year, to great ratings success.

An older, related concept is the summer replacement, which debuts between May and August, when a network's marquee programs go on hiatus. Summer replacements tend to be lower-profile shows with either low budgets or minimal prospects for renewal.

...
Wikipedia