remarkable effect; miraculous efficacy
Used for a surprisingly good or almost miraculous effect, often of a medicine, treatment, or method. Somewhat literary or formal.
This new drug produced a remarkable effect.
その治療法は奇効があると評判だ。
That treatment is reputed to have miraculous efficacy.
Compound of 奇 (ki, 'strange, wondrous') and 効 (kō, 'effect, efficacy'). The exact historical derivation is uncertain, but the word is conventionally associated with a remarkably good result.