In general, a threshold is a fixed location or value where an abrupt change is observed. It may also have the following meanings:

• technical meanings:
  • in architecture, a threshold is a strip of wood, metal, rubber or stone located on the floor in a doorway following the line of division between the two spaces linked by the doorway.
  • in aviation, a threshold is the end of a runway over which an airplane first passes when landing.
  • in biology, an action potential is initiated if the membrane potential is depolarized to the threshold potential. See: Action potential.
  • in chemistry, threshold energy is a synonym for activation energy, the minimum energy necessary for a specific chemical reaction to occur.
  • in telecommunication, the term threshold has the following meanings:
    1. The minimum value of a signal that can be detected by the system or sensor under consideration.
    2. A value used to denote predetermined levels, such as those pertaining to volume of message storage, i.e., in-transit storage or queue storage, used in a message switching center.
    3. The minimum value of the parameter used to activate a device.
    4. The minimum value a stimulus may have to create a desired effect.
    • Source: from Federal Standard 1037C and from MIL-STD-188
  • in electrical engineering and machine learning, a threshold circuit or unit is a one-input, one-output device or object that implements a Heaviside step function; the threshold value corresponds to the transition in this function between one and zero.
  • in cryptography, a cryptosystem is called a threshold one, if in order to decrypt a secret a number of parties that hold a key share exceeding the threshold is required.
  • in elections, election threshold refers to the minimal amount of votes necessary to win representation.
  • in optics, the lasing threshold is the minimum power level at which a laser starts to operate.
  • in psychology, sensory threshold is one of the nine aspects of temperament. The minimum amount of stimulus energy necessary to elicit a sensory response.
  • in occupational safety and health, see ACGIH and its Threshold Limit Values (TLV).
• popular culture:
  • Threshold (book) is the name of a science-fiction book by David R. Palmer.
  • Threshold (book 2) is also the name of a fantasy book by Sara Douglass.
  • name of progressive metal band Threshold (band) from England.
  • in the video game "Halo", Threshold (fictional planet) is the planet around which the ringworld Halo orbits.
  • the Sci Fi Channel aired a movie in 2003 entitled Threshold. ^*
  • Threshold (TV series) is also the name of a science-fiction television series that began airing in 2005 on CBS.
  • "Threshold" was an episode in Voyager.
  • Threshold RPG is a text-based roleplaying MUD game.
  • Threshold (Wildstorm) is a comic book character from Wildstorm

The word "threshold" is derived from O.E. þrescold, þærscwold, þerxold "doorsill, point of entering," first element related to O.E. þrescan (see thresh), with its original sense of "tread, trample." Second element of unknown origin and much transformed in all the Gmc. languages; in Eng. it probably has been altered to conform to hold, but the oft-repeated story that the threshold was a barrier placed at the doorway to hold the chaff flooring in the room is mere folk etymology. Cognates include O.N. þreskjoldr, Swed. tröskel, O.H.G. driscufli, Ger. dial. drischaufel.

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