Smart object
A Photoshop layer that holds embedded source content, letting you place or replace artwork non-destructively — the mechanism that makes a mockup reusable.
Also called: smart layer
A smart object is a container layer in Photoshop. Instead of editing pixels directly, you choose “Edit Contents”, paste your artwork, save, and the parent document updates. In a mockup this is what you replace: double-click the smart object, drop in your logo or label, and it appears on the scene at the right scale and perspective.
Because the transform is stored on the smart object, your original artwork is never distorted — only its on-screen representation is. A reliable, clearly-named smart object (with a sensible transform and a displacement map above it) is the single biggest factor in how usable a mockup is.