The Soap Opera
A serial that ran every weekday, in the middle of the day. This form, made by American radio, took its name not from its content but from the sponsor's product. The soap companies were the main backers, so: soap opera.
What is nice is that the explanation is written down in a paper of the time. A newspaper in 1938 reported that the name had come from among the radio actors, and explained that since the soap manufacturers were the first to make large-scale use of these afternoon serials, the actors, being actors, made the name stick. It is not an etymology invented afterwards.
The daytime serial begins in 1930, with a fifteen-minute family story on a Chicago station. The form crossed from radio to television, and Guiding Light ran for more than seventy years, from radio in 1937 to the end of the television show in 2009. The pattern of a story continuing daily in a fixed slot later took root, in a different shape, in Japan's morning serials (→ Asadora).