A Lyot stop is a mask in the reimaged optical pupil after a coronagraphic focal plane mask. For an unobstructed circular entrance pupil, it usually transmits a smaller concentric disk. In a classical Lyot coronagraph, much of the residual stellar field lies near the optical pupil boundary; the Lyot stop rejects that field before the final image is formed. It also rejects useful off-axis light, so choosing its opening involves a coronagraph throughput tradeoff. [ref]
https://ao.jpl.nasa.gov/Publications/Technical/2001ApJ.sivaramakrishnan.AO.pdf. Sivaramakrishnan et al., 2001, §2.1
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A Lyot stop is a specific optical element used in various types of imaging systems, particularly in the context of astronomical telescopes and spectrometers. Named after the French physicist Paul Lyot, it functions as a spatial filter to improve the quality of the light that passes through by blocking unwanted portions of the light wavefront. In practical terms, the Lyot stop is typically placed at or near the focal plane of the telescope or optical system.