non-defining relative clause
non-restrictive clause
non-restrictive relative clause

A restrictive, or defining, relative clause modifies the meaning of its head word (restricts its possible referent), whereas a non-restrictive or non-defining relative clause merely provides supplementary information.

It is used, for example, to distinguish between a defining relative clause such as He was waving to the girl who was running along the platform and a non-defining relative clause such as He was waving to the girl, who was running along the platform.