High base refers to the height of the internal base wad of a shotshell.

High base is commonly misused as a term referring  to a shotshell with a high metal cup. Although its is also commonly called ‘high base’ or ‘high brass,’ the proper term to describe a shotshell with a high metal cup is ‘high cup‘.

Footnotes / Sources:

“Glossary ⇒.” Sporting Arms and Ammunition Manufacturers’ Institute. Ed. SAAMI’s Technical Committee. Web. 15-18 Jan. 2011.