Philately (/fɪˈlætəliː/ fi-la-tə-lee) is the study of stamps and postal history and other related items. Philately involves more than just stamp collecting, which does not necessarily involve the study of stamps. It is possible to be a philatelist without owning any stamps.[1] For instance, the stamps being studied may be very rare, or reside only in museums.

There’s also a catalog issued by the USPS of same name (PDF, I think there’s a newer one out): https://www.usps.com/stamp-collecting/assets/pdf/usa_philatelic_catalog.pdf