The impurity that remains during the settlement of insoluble substances is called :
Sediment
In decantation and sedimentation method, the insoluble solid substances settle down after the mixture is left undisturbed and the clear liquid is left standing. This is called supernatant liquid. The solid substances that settle down are called sediments. These methods are usually preferred to separate a heterogeneous mixture which is made up of insoluble solid in liquid.