(empty radius-range within a disk separating it into rings)
A disk gap is a gap within a disk, dividing it into two concentric
rings.
These are found in some protoplanetary disks and are considered evidence
of a planet, or protoplanet, the latter indicating
planet formation.
I presume ALMA, with its ability to image protoplanetary disk
detail, has made the discovery of disk gaps far more frequent.