A bioscan was a process in which an organism's biological structure – both external and internal – was scanned, normally for medical purposes.
An unidentified hostile vessel scanned the crew of Enterprise NX-01 via a submolecular bioscan in 2151 for useful compounds in their lymphatic systems. (ENT: "Fight or Flight")
In the script for "Fight or Flight", a submolecular bioscan is described as consisting of "a low-pitched scanning noise... followed by a thin curtain of modulating light that sweeps across [an area]."
A bioscan of the Novan Nadet by Doctor Phlox the same year revealed that she was ill with lung cancer. (ENT: "Terra Nova")
While aboard the ECS Fortunate, also in 2151, a bioscan conducted by T'Pol using a Vulcan scanner picked up a Nausicaan life sign aboard the freighter. The bioscan was able to determine that the Nausicaan was also injured. (ENT: "Fortunate Son")
Doctor Beverly Crusher performed several bioscans on Susanna Leijten and Geordi La Forge when they were affected by the Tarchannen III species in 2367. Each scan proved inconclusive and no infections were detected. A short time later, however, both were transformed by the disease into Tarchannen III lifeforms. (TNG: "Identity Crisis")
Doctor Crusher ran a bioscan on Berlinghoff Rasmussen to verify his identity in 2368. The readings indicated that he was perfectly Human. (TNG: "A Matter of Time")
Geordi La Forge checked the bioscans of himself, Captain Picard, and Deanna Troi in 2369 after Troi told the others about her unusual experience seeing the others "frozen in time". (TNG: "Timescape")
Captain Kathryn Janeway ordered full bioscans on Harry Kim and Derran Tal in 2375 after Kim began exhibiting symptoms of olan'vora in 2375. (VOY: "The Disease")