Open-collector output
also known as: Open-drain
An output that can only pull LOW — its transistor sinks current to ground or lets the line float. The HIGH state comes from an external pull-up. Open-drain is the MOSFET version of the same idea.
In practice
It looks broken on first encounter: 'my comparator never goes high.' It isn't broken; it's missing its pull-up. The payoff is that many open-collector outputs can share one wire safely (wired-AND) — which is why I²C, interrupt lines, and fault flags use it.