A "serial" is a publication or broadcast that is released piece-by-piece at regular intervals, such as a TV miniseries. The word "serial" is also used in adjective form to describe anything occurring in a series.
One of the most well-known uses of the word "serial" is in the term "serial killer," which describes someone who commits multiple murders in separate occurrences over a period of time. In this example, the adjective "serial" does not describe the multiplicity of the murders but the fact that the murders happen on more than one occasion. "Serial" also refers to magazine articles, radio broadcasts, podcasts or TV shows that only release one part of the story each week.