Open drain
An output that can only pull a line to ground, never drive it high — a pull-up resistor handles that part.
A normal push-pull output has one transistor to the supply and one to ground. Open drain keeps only the bottom one, a MOSFET whose drain is left unconnected. When it lets go, a pull-up resistor lifts the line. The older bipolar version is called open collector.
It sounds like half an output, and that is precisely the point. Several devices can share one wire without shorting each other out: if anyone pulls low the line is low, otherwise it is high. This wired-AND behaviour underpins I²C and shared interrupt lines. Because the pull-up can go to a different voltage than the chip's own supply, open drain also doubles as a cheap level shifter between 3.3 and 5 volts. The catch is slow rising edges.