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A fold mirror (or folding mirror) in a reflector telescope is a flat mirror that redirects the light for convenience in the design, e.g., to place the focal plane in a convenient location, such as for a large instrument or for an eyepiece. A Newtonian telescope differs from a prime focus telescope in that it incorporates a fold mirror to redirect the light sideways. Other telescope type terms (e.g., Cassegrain reflector) are commonly used whether or not a fold mirror is added for convenience.
Note that the term folding mirror could clearly be used with another meaning, for mirrors such as that of the James Webb Space Telescope that are capable by design of being folded in storage before deployment.