major (train) station; main station
main part; key part; head (of a phrase)
main parts; main components
primary business; primary function; primary focus
major newspaper; leading newspaper; widely circulated newspaper
primary factor; main cause
principal movement (of an earthquake); principal shock
prices of staple commodities
essential elements of information
key customers; key customer; major clients; major client
major histocompatibility complex; MHC
key industry; major industry