A monocle was a type of glass lens used for only one eye.
The First Doctor used a monocle when he encountered the Sensorites, (TV: The Sensorites) Fomal, (PROSE: The Sons of the Crab), the Korads (PROSE: Peril in Mechanistria), the Voord (PROSE: The Fishmen of Kandalinga) and the Testimony Foundation. While in the ruins of Villengard, he used his monocle to examine a discarded eyestalk. (TV: Twice Upon a Time)
The War Lord von Weich used a monocle to implant suggestions into the minds of his human subordinates. (TV: The War Games)
The computer Gus used the image of a monocle as its avatar. (TV: Mummy on the Orient Express)
The Seventh Doctor wore a monocle as part of his Reichsinspektor General guise. (PROSE: Timewyrm: Exodus)