The concentration of the substrate at which half of the active sites of the enzyme are occupied by the substrate is known as 

${\mathrm{K}}_{\mathrm{m}}$, the substrate concentration at which the chemical reaction catalysed by an enzyme attains half its maximum velocity.

The catalytic efficiency of two different enzymes can be compared by the Km value.

Km value is inversely proportional to the affinity of the enzyme to its substrate.