Multipurpose scientific and technical instrument. This handheld device incorporates sensors, computers, and recorders in a convinient, portable form. Several models of tricorders have been featured state-if-the-art sensing technology, and all have been an essential part of starship missions and operations. Specialized tricorders are available for specific engineering, scientific, and medical applications. Standard-issue Starfleet tricorders were unable to detect subspace phenomena in 2366 (TNG: The Bonding). They were also incapable of sensing neutrino emissions (TNG: The Enemy).
In the fictional Star Trek universe, a tricorder is a handheld device used for scanning an area, interpreting and displaying data from scans to the user, and recording information to isolinear chips.
Three primary variants of the tricorder were issued in Star Trek's Starfleet. The standard tricorder is a general-purpose device used primarily to scout unfamiliar areas. The medical tricorder is used by doctors to help diagnose diseases and collect bodily information about a patient; the key difference between this and a standard tricorder is a detachable hand-held high-resolution sensor array stored in a compartment on the front of the tricorder when not in use, making it slightly bigger than a standard tricorder. The engineering tricorder is fine-tuned for starship engineering purposes. There are also many other lesser-used varieties of special use tricorders.
According to Dr. Julian Bashir (from Star Trek: Deep Space 9), while medical tricorders are very good at scanning living people, they are not very good at scanning dead ones. Evidently this is the first lesson taught at Starfleet medical school.
The tricorder of the 23rd century was a heavy, black, rectangular device with a small screen and a shoulder strap. The 24th century unit is a small, gray, square model with a flip-out panel to allow for a larger screen. This design was further refined later on with a slightly more angular appearance that was seen on most of the TNG-era movies as well as later seasons of DS9 and Voyager. The medical variants of this model had the detachable scanner stored in the bottom of it when not in use as opposed to the front, giving this model the same overall profile regardless of its intended use. Later in the post next generation era, Star Trek Nemesis and Star Trek: Elite Force II, a newer tricorder is introduced. It is flatter, and looks more like a palm pilot, with a small flap that flips up on the top and a large touch screen interface.