Instance segmentation gives each separate object its own pixel mask, so two overlapping people get two distinct silhouettes. It is detection and segmentation at once, used where you must isolate and act on each object individually.
Definition
Pixel masks that also separate individual objects — not just 'person pixels' but 'person 1' vs 'person 2'. Combines detection with segmentation.
Instance segmentation gives each separate object its own pixel mask, so two overlapping people get two distinct silhouettes. It is detection and segmentation at once, used where you must isolate and act on each object individually.