What is a synonym for periscope?

There isn't a single perfect synonym for "periscope," as it refers to a specific device. However, depending on the context, you could use:

For the general idea of seeing over something:

* Observation device

* Viewing device

* Viewing instrument

* Lookout

For the specific function of seeing from a concealed position:

* Submarine viewer (if referring to a submarine)

* Observation tube (if referring to a simple tube for seeing over something)

* Telescopic sight (if referring to a device used for aiming)

For the technical aspect of using mirrors:

* Mirror system

* Optical system

The best synonym will depend on the specific situation and the intended meaning.