a decisive step role / battle, etc is an action, event, etc. that has a powerful effect on the final result of sth Examples Waterloo was the decisive battle of the entire war He played a decisive role in the recent peace process.a decisive step / role / battle, etc — is an action, event, etc. that has a powerful effect on the final result of sth. Examples • Waterloo was the decisive battle of the entire war. • He played a decisive role in the recent peace process