Im not sure if there is a definitions page but primary and secondary refer to the fermenters or the steps that are used in two stage fermentation. First you ferment in your primary and then you go to the secondary after getting identical readings with your hydrometer 3 days in a row.
However most people leave their beer in a primary for the majority if not ALL of the time they ferment, which should be at least 3 weeks. People mostly use secondaries for clearing out their beer or adding fruit.