1. noun /ˈsɪə.riːz/
Friends was one of the most successful television series in recent years.<!note that the italicisation is correct here
b) A television or radio program which consists of several episodes that are broadcast in regular intervals
The third series of Friends aired from 1996 to 1997.
2. adjective /ˈsɪə.riːz/
Connected one after the other in a circuit.
You have to connect the lights in series for them to work properly.
Wikipedia foundation.