effectiveness

The word “effectiveness” means the degree to which something achieves the result it is meant to. In plain terms, how well something actually works at doing what it's supposed to do.

“Effectiveness” is pronounced ih-FEHK-tihv-nihs (/ɪˈfɛk.tɪv.nɪs/ in IPA).

Here is “effectiveness” used in a sentence: “Studies measured the effectiveness of the new vaccine.”

“Effectiveness” is a noun.

“Effectiveness” is a fairly common word in modern English, usually learned at around the B2 level.

Effectiveness is the degree to which something achieves the result it is meant to; efficiency is how little time, money, or effort it wastes in doing so. A treatment can be effective but inefficient — it works, but slowly and expensively — and a process can be efficient yet ineffective.

Efficacy is the power of something to produce its intended result under ideal or controlled conditions, such as a clinical trial. Effectiveness is the degree to which it achieves that result in real-world use, which is why a medicine's trial efficacy can be higher than its everyday effectiveness.

The adjective is effective, meaning achieving the intended result, and the related adverb is effectively. Effectiveness is the noun that names the quality, as in "the effectiveness of the new policy."