In a direct impact, loss in kinetic energy is given by ΔK=M1M22M1+M2⋅V1−V221−k2 with usual notations (except k). The quantity k will have dimensional formula
M0L2T−2
MLT−1
M0L0T0
M0LT−1
It is clear that k should be dimensionless
1−k2=M0L0T0 Hence, [k]=M0L0T0
Actually k is the coefficient of restitution.