Rapid.
Greek takhus, swift.
In medicine, tachycardia (Greek kardia, heart) is an abnormally rapid heart rate; tachyphylaxis (Greek phulaxis, act of guarding) occurs when repeated doses of a drug cause responses that are progressively and rapidly less effective; tachypnoea (US tachypnea) (Greek pnoē, breathing) is abnormally rapid breathing. A tachyon is a hypothetical particle that travels faster than light.
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