What does "PSE" mean in PoE / Power over Ethernet context?

PD means Power sourcing equipment. This refers to a device which injects power in a PoE port - in other words, it provides Power over Ethernet as opposed to a PD (powered device) that consumes power injected by a Power sourcing equipment device.

Also see:What does “PD” mean in PoE / Power over Ethernet context?