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Fast

  1. (dated) Firmly or securely fixed in place; stable. [from 9th c.]

Synonyms: firm, immobile, secure, stable, stuck, tightAntonym: looseHyponyms: bedfast, chairfast, colorfast, fail-fast, lightfast, shamefast, soothfast, steadfast

  1. Firm against attack; fortified by nature or art; impregnable; strong. Synonyms: fortified, impenetrableAntonyms: penetrable, weak
  2. (of people) Steadfast, with unwavering feeling. (Now mostly in set phrases like fast friend(s).) [from 10th c.]
  3. Moving with great speed, or capable of doing so; swift, rapid. [from 14th c.]

Synonyms: quick, rapid, speedy

    1. (nuclear physics, of a neutron) Having a kinetic energy between 1 million and 20 million electron volts; often used to describe the energy state of free neutrons at the moment of their release by a nuclear fission or nuclear fusion reaction (i.e., before the neutrons have been slowed down by anything).
  2. (computing, of a piece of hardware) Able to transfer data in a short period of time.
  3. Deep or sound (of sleep); fast asleep (of people). [16th-19th c.] Synonyms: deep, soundAntonym: light
  4. (of dyes or colours) Not running or fading when subjected to detrimental conditions such as wetness or intense light; permanent. [from 17th c.]

Synonym: colour-fast

  1. (obsolete) Tenacious; retentive.
  2. (dated) Having an extravagant lifestyle or immoral habits. [from 18th c.]
  3. Ahead of the correct time or schedule. [from 19th c.]

Synonyms: ahead, gain (as in “the clock is gaining x minutes per hour/day”) Antonyms: behind, slow

  1. (of photographic film) More sensitive to light than average. [from 20th c.]
Usage notes

In the context of nuclear reactors or weaponry, fission-spectrum neutrons (neutrons with the spectrum of energies produced by nuclear fission) are frequently referred to as fast neutrons, even though the majority of fission-spectrum neutrons have energies below the 1-million-electron-volt cutoff.

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