What is exact meaning of reduce in useReducer of react

Question

As a freshman in react and Js,

I'm always wondering where 'reducer' come from.

It seems clear for me, that useState means using states.

but not for reducer.. reducer use dispatch and state, but has name of 'reducer'

i can find reducer in English means for 'pipefitting that joins two pipes of different diameter' or 'a substance capable of bringing about the reduction of another substance as it itself is oxidized'

Whats the exact meaning of reduce and reducer in useReducer??