Cistern and Spyhole

Cistern noun - A sac or cavity containing fluid especially lymph or cerebrospinal fluid.

Spyhole and cistern are semantically related. In some cases you can use "Spyhole" instead a noun "Cistern".

Spyhole noun - A hole (in a door or an oven etc) through which you can peep.

Cistern and spyhole are semantically related. Sometimes you can use "Cistern" instead a noun "Spyhole".