This is the 7636th most frequent Japanese word.
効力
Effectiveness; efficacy; validity.
Here, '効力' is used to express the binding effect or legal effectiveness of the contract.
この契約には効力があります。
This contract has validity.
In this sentence, '効力' refers to the power or effectiveness of the medicine when under the influence of time.
薬の効力は時間が経つと弱まることがあります。
The effectiveness of medicine may weaken over time.
'効力' here emphasizes the impactful force or influence of his words on others.