Undercut: An undercut is a feature that a straight-pulling mould or a straight cutting tool cannot reach or release, such as a hook, a side hole or a groove hidden behind another face.
The word has slightly different meanings depending on the process, but the idea is the same: a shape that is hidden from the direction the tool or mould moves.
In moulding, the two halves of a mould open in a straight line. A snap-fit hook, a hole through a side wall or a groove around a cylinder locks the part in the mould. Releasing it needs a side action, a lifter or a collapsible core, all of which add to the cost of the tooling.
In CNC machining, a standard end mill cuts straight down from above. A groove under a lip, or a pocket wider at the bottom than at the top, cannot be reached. It needs a special undercut cutter (such as a T-slot or lollipop cutter), another setup, or 5-axis machining.
In 3D printing undercuts are usually no problem, which is one reason printing suits complex parts.
Example: a moulded clip has one hook. Removing the hook, or turning it to face the direction of draw, may let a simple two-part mould do the job instead of one with a side action.
Undercuts are often designed in without the designer realising, because they look harmless in CAD. A DFM check that looks for faces hidden from the tool direction catches them early. Read design for CNC machining for how to avoid them.
Go deeper: Design for CNC machining: the rules that cut cost