Once upon a time, the coulomb was defined as the fundamental unit of charge, and the
ampere was a derived unit proportional to charge (in coulombs) and inversely proportional to time (in seconds). More specifically, one ampere is equal to a charge transfer rate of one coulomb per second. So, it is a rate -- a charge-transfer rate.
Nowadays, the ampere is considered the fundamental unit, and the coulomb is the derived unit. In other words, the coulomb is the amount of charge delivered in one second by a current of one ampere.
The coulomb has
never ever been considered a Base Unit in either the SI or earlier metric systems. The ampere has
always been considered a Base Unit. Prior to 1948, the ampere was defined in terms of its chemical effect; after 1948 it was defined in terms of its magnetic effect -i.e. in terms of the force between two, parallel, current-carrying conductors. So, the ampere has
never been defined as an unit of rate. On the contrary, the coulomb is a Derived Unit, based on the ampere and the second.