Motivator (noun)

1. A person or thing that motivates or inspires others to take action or to achieve a goal.
2. Something that provides a reason or stimulus for doing something.

Origin:

from the verb "motivate" + -or (noun-forming suffix).

Examples:

1. He was a great motivator and always encouraged his team to do their best.
2. She was the motivator behind the charity event.
3. Money was the motivator for him to take the job.
4. The motivational speech was a success and the team was inspired to perform better.
5. His leadership style was a motivator for his team to achieve success.