Mask

Draw an ellipse over somebody's face, apply a blur, and only the face blurs. That ellipse is a mask. Masks can be simple shapes or drawn point by point, and feathering the edge by 20 pixels stops the boundary showing. When the subject moves, the mask has to move too, which is the job tracking does.

Creators meet them most often for blurring a face or a number plate, hiding a logo, and split-screen shots where the same person appears twice. The neighbouring word is matte, which is the same idea held as a greyscale image rather than a drawn shape.