Plain English Meaning
Your heart beating faster than it should when you are at rest.
Full Definition
An abnormally rapid heart rate, typically defined as a resting heart rate above 100 beats per minute in adults. It can be caused by physical exertion, stress, low oxygen levels, exposure to certain gases such as carbon dioxide, or underlying medical conditions.
Context Anchor
Seen in safety warnings about carrying dry ice in an aircraft, where dry ice can turn into carbon dioxide gas and affect the people on board.
Derivation
From the Greek 'tachys' meaning 'fast' and 'kardia' meaning 'heart.' The same 'tachy-' root appears in 'tachometer,' the cockpit instrument that measures engine RPM — both relate to measuring how fast something is turning .