reducer: any function with the signature (state, action) -> newState (ie, any function that could be used as an argument to Array.prototype.reduce)root reducer: the reducer function that is actually passed as the first argument to createStore. This is the only part of the reducer logic that must have the (state, action) -> newState signature.slice reducer: a reducer that is being used to handle updates to one specific slice of the state tree, usually done by passing it to combineReducerscase function: a function that is being used to handle the update logic for a specific action. This may actually be a reducer function, or it may require other parameters to do its work properly.higher-order reducer: a function that takes a reducer function as an argument, and/or returns a new reducer function as a result (such as combineReducers, or redux-undo)