Hollowing: Hollowing is removing the solid interior of a 3D model so the printed part is a shell, saving material and weight, mostly in resin printing.
Resin printers cure every bit of a solid part, so a chunky solid figure uses a lot of costly resin. Hollowing turns it into a shell with a set wall thickness, often a couple of millimetres for resin. A hollow resin part needs drain holes. Without them, uncured resin stays sealed inside, and the closed shell can act like a suction cup against the vat during printing, then leak or crack later.
What it means for the price
Hollowing cuts resin use, sometimes sharply for large solid parts, and lighter layers peel more easily from the vat. Against that, it adds preparation time, and the shop must check that the walls are thick enough and the holes are in the right places. In FDM, infill does a similar job automatically.
Example: a solid 200 ml figurine hollowed to a 2 mm wall might need around 60 ml of resin, depending on its shape. The figures are an example.
Common confusions
A hollow part without drain holes has a trapped volume, which is a defect, not a saving. Hollowing is not the same as thinning walls below the minimum wall thickness, which risks breakage. Read what drives resin printing cost.
Go deeper: Resin printing cost: how to price SLA, DLP and MSLA prints