There surprisingly no proper definition of this term available on google. There are many research papers that are based on "finding the optical counterparts".
What exactly are optical counterparts and how to "find" them?
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Observations can be made using different parts of the electromagnetic spectrum. Many of them not within the range of the spectrum that can be made with optical telescopes. Other messengers than photons (e.g gravitational waves, neutrinos,...) can also be observed.
Optical counterparts are observations made in the spectrum observable using optical telescopes that can be matched up with observations made using other observational methods, including radio telemetry, Gamma ray detectors, and gravity wave detectors.
Observers are interested in lining up observations made up of any combination of observational methods as this provides the opportunities to identify correlations that are a rich source of new discoveries.