noun
effectiveness; practical efficacy
Refers to whether something actually produces the intended result in practice, often used in policy, legal, or technical contexts.
この政策の実効性が問われている。
The effectiveness of this policy is being questioned.
法律の実効性を高める必要
We need to enhance the practical efficacy of the law.
効果 is a general term for effect or result, whereas 実効性 specifically refers to the quality of being effective in practice.
Compound of 実効 (じっこう, 'actual effect') + 性 (せい, '-ness, -ity'). The exact historical derivation is uncertain, but the word follows a common Sino-Japanese pattern for abstract nouns.