Mill-turn: A mill-turn machine is a CNC lathe with powered milling tools, so it can turn a part and mill flats, holes and pockets on it without moving it to another machine.
A plain lathe only cuts while the part spins. A mill-turn centre adds live tooling (powered spindles on the turret) and a C axis that indexes or rotates the part precisely, and often a Y axis and a second spindle to finish the back of the part. A part with a turned body and a cross-hole, a hex or a keyway can come off the machine complete.
What it means for the price
Mill-turn machines carry a higher hourly rate than plain lathes. But removing a second operation on a separate mill saves a setup, the handling between machines, and the risk of features on the two machines not lining up. For batches of turned parts with a few milled features, finishing in one machine is often cheaper overall. For a simple round part, a plain CNC lathe is cheaper.
Example: a shaft with a keyway could take 4 minutes on a lathe plus 6 minutes on a mill (with its own setup), or 7 minutes on a mill-turn centre. The figures are an example.
Common confusions
Mill-turn is not the same as a five-axis mill with a turning table, though the two families overlap in what they can make. Fewer operations also means fewer cycle time hand-offs. Read CNC milling vs turning.
Go deeper: CNC milling vs turning: which process for which part