Pivotal (adjective)

1. Of crucial importance in relation to the development or success of something else.
2. Crucial in determining the outcome of a situation.

Origin:

Early 17th century (as a noun denoting a pivot or central point): from pivot + -al.

Examples:

1. The meeting was pivotal in the decision-making process.
2. The role of the mentor was pivotal in the success of the project.
3. The election results were pivotal in determining the future of the country.
4. The discovery of DNA was pivotal in the field of genetics.
5. The negotiations were pivotal in determining the fate of the company.