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A Herschelian telescope is a type of off-axis reflector telescope with one mirror: a primary mirror that reflects toward one side. The type is named for William Herschel who, among other things, created very large telescopes (for his day) using this design, the largest with a 1.2-meter aperture. Telescope mirrors of that time had low reflectivity, a motivation for avoiding additional mirrors. Herschel used parabolic mirrors and I'm guessing the paraboloid's axis was slightly tilted to the incoming rays. He reduced geometrical aberrations by using a long focal length, making the telescopes were very long. Off-axis telescopes (and equivalent antenna reflectors) have now become common, e.g., for radio telescopes, now that computers help create and validate the designs, but in many cases, they incorporate additional mirrors/reflectors.