In mathematics, Nambooripad order[1] (also called Nambooripad's partial order) is a certain natural partial order on a regular semigroup discovered by K S S Nambooripad[2] in late seventies. Since the same partial order was also independently discovered by Robert E Hartwig,[3] some authors refer to it as Hartwig–Nambooripad order.[4] "Natural" here means that the order is defined in terms of the operation on the semigroup.
In general Nambooripad's order in a regular semigroup is not compatible with multiplication. It is compatible with multiplication only if the semigroup is pseudo-inverse (locally inverse).