The word is also used for the accumulation itself, fluids and pressure included. In that sense a reservoir has contacts, a pressure, and a drive mechanism, not only a lithology.
It is not the same thing as a trap. The trap is the configuration that stops migration. The reservoir is the part of that configuration that can hold and flow. A sealed structure full of shale is a trap with no reservoir. A thick sand that is open updip is a reservoir rock with no trap.