Reactance may refer to:
- Electrical reactance, the opposition to a change in voltage due to capacitance (capacitive reactance) or in current due to inductance (inductive reactance)
- Magnetic reactance, a similar effect in magnetism
- Reactance (psychology), an emotional reaction to pressure or persuasion that results in the strengthening or adoption of a contrary belief
- Reactivity (chemistry), the rate at which a chemical substance tends to undergo a chemical reaction
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