means In electrical circuits, reactance is the opposition to alternating current that comes from inductors and capacitors, storing and releasing energy rather than dissipating it like resistance does — measured in ohms.

from A 19th-century technical coinage built from 'react' plus the suffix '-ance' (denoting a state or quality), formed on the same pattern as 'resistance' and 'capacitance.' 'React' itself comes from Latin 're-' ('back, again') and 'agere' ('to do, drive, act') — so at root it means 'to act back.' The word was adopted by electrical engineers as they worked out how circuits push back against changing currents, a fitting name for a force that responds to change by acting against it.