Refers to a counter-effect or backlash, often in scientific, technical, or figurative contexts where an action produces an opposing result.
As a reaction to this medicine, drowsiness may occur.
As an adverse effect of the new policy, confusion spread instead.
副作用 specifically means 'side effect', especially of medicine. 逆作用 is broader and can refer to any counterproductive result.
Compound of 逆 (reverse, opposite) and 作用 (action, effect). The exact historical derivation is uncertain, but the word follows standard Sino-Japanese compounding patterns.