Transiting Exoplanet Survey Satellite (TESS) is a spacecraft on a mission to survey 2,000,000 nearby stars for transiting planets, launched in April 2018 for a 2-year primary mission. It uses four wide-angle telescopes and stores three months' data, allowing it to prioritize transmission to ground stations of promising data. It aims for stars brighter than 12 magnitude, including G-type stars and K-type stars, as well as nearby M dwarfs, small M-type stars. After the 2-year primary mission, extension was approved, TESS began additional observations in July 2020. The satellite remains usable and useful and additional extensions are probable. The TESS project began as an offshoot of the High Energy Transient Explorer (HETE) gamma-ray burst series, originally called HETE-S (for Hot Exoplanet Transit Explorer-Survey), aiming to adapted HETE technology, making use of the ongoing improvements in optical CCDs to perform an all-sky survey for exoplanets.
TOI stands for TESS object of interest, a term for a possible planet, i.e., the location of observations that appear to reveal a transiting planet. TFOP (for TESS Follow-up Observation Program) is an effort to investigate TESS discoveries, e.g., to confirm whether a TOI is a planet. As of 6/2021, TESS has produced over 3300 candidates (TOIs) among its target stars including at least 760 apparently between Earth and Neptune in radius, and 129 confirmed to be planets through follow-up observation.
The terms TESS Duo and TESS-bis have been used the a concept of launching a second TESS satellite. TESS spots many transients (e.g., supernovae) as well as SSSBs, and an additional TESS, taking advantage of the existing design, could offer immediate confirmation of apparent transients as well as parallax regarding solar system orbits.