A visual sensor was part of the sensor technology which was placed on board starships and allowed visual frames of a specified region.

In 2152, moments after Captain Jonathan Archer found an alien device in launch bay 2, he activated visual sensor J-15 and left a message for a group of unidentified aliens, who had placed the device on board. (ENT: "Silent Enemy")

Voyager had long range visual sensors, which it used in 2374 to image a relay station in the Hirogen communications network. (VOY: "Hunters")

The sequence of events in the episode makes it clear that the range at which these sensors could be used was less then two light years.
The term "visual sensors" was once referred to in dialogue from the first draft script of ENT: "Shadows of P'Jem" (written while the episode had the working title "Untitled Andorians Return"). In that script, Phlox advised T'Pol to "keep [her] visual sensors on alert" for a forthcoming going-away party that the crew might throw her. This line was changed to Phlox instead advising T'Pol to "keep on alert" for the party.
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