The proportionality constant of rate equation is called rate constant. It is also called specific rate constant as it has specific value for specific reaction.
Let us consider a chemical reaction-
R → P
Rate = − d[R]/dT = + d[P]/dT
rate of reaction ∝ [R]
so, the rate law equation: − d[R]/dT = k[R]
where K is rate constant
When [R] = 1, then-
− d[R]/dT = k
So, the rate constant k is the rate of the reaction at unit molar concentration of the reactant.