Question

what does it mean by SI unit that is a coherent system?

Answer 1

Equivalently, it is a system in which every quantity has a unique unit, or one that does not use conversion factors. A coherent derived unit is a derived unit that, for a given system of quantities and for a chosen set of base units, is a product of powers of base units with the proportionality factor being one.

Answer 2

A coherent derived unit is defined as a derived unit that, for a given system of quantities and for a chosen set of baseunits, is a product of powers of baseunits with no other proportionality factor than one.