Named for Irving Feinberg the property master of Star Trek TOS. Colloquially a Feinberger was any prop. It has come to mean Dr. McCoy's cylindrical medical scanner.
The medical scanner is a quick non-invasive method to get a patient’s vital signs. Heartbeat, respiration, temperature, blood pressure. All the things they check when you go to the doctors office, just done in five seconds without touching you. (Where do I get one?)
Not as accurate or informative as a bio-bed but far more portable and quick. The Medical Scanner can interface with the tricorder to get more accurate and complete readings.