Mould cavity: A mould cavity is the hollow in a mould that forms one part; a multi-cavity mould has several, making several parts in each shot.
The cavity is the negative of the part, split between the two halves of the mould. A single-cavity mould makes one part per cycle; two-, four- or eight-cavity moulds make that many. A family mould holds cavities for different parts of the same product, such as a lid and a base.
What it means for the price
More cavities cost more up front, because the mould is larger and more complex, but each part is cheaper, because the machine time of one shot is shared. The right number depends on quantity: for a few thousand parts a single cavity is often cheapest overall, while for hundreds of thousands more cavities usually win. A quote should show that trade, not choose silently.
Example: a single-cavity mould at €8,000 making parts at €0.40 each, against a four-cavity mould at €18,000 making parts at €0.12 each, break even at about 35,700 parts. The figures are an example.
Common confusions
“Cavity” also names the half of the mould that forms the outside of the part, as opposed to the core that forms the inside. Both meanings are common. See tooling and read what drives injection moulding cost.
Go deeper: Injection moulding cost: tooling, cycle time, resin and the break-even