効果【こうか】
effect, effectiveness, efficacy, result, effects (e.g. sound effects, visual effects, special effects)
有効【ゆうこう】
valid, effective, yuko (judo)
効率【こうりつ】
efficiency
無効【むこう】
invalid, void, ineffective, unavailable
時効【じこう】
statute of limitations, lapse of rights after a period of time, prescription (including acquisitive and extinctive prescription), becoming invalid or void after a set time, ageing, aging
効力【こうりょく】
effect, efficacy, validity, potency
効き目【ききめ】
effect, virtue, efficacy, impression, one's dominant eye
逆効果【ぎゃくこうか】
opposite effect, adverse effect, backfiring