An achievement or promise is fulfilled when it's reached or kept.
Fulfill is often used to describe things that have been completed or carried out as planned. When you say "I've fulfilled my dream of traveling around the world," you mean you've finally done it, after years of saving and planning. People also use fulfill to talk about promises made in a business setting. If a company says they'll deliver a certain product by a certain time, and they do, then they've fulfilled their commitment.
How common is "fulfill"?
Word fulfill is considered uncommon in modern English. It has a balanced usage among all categories: speech, fiction, newspapers and academic texts.
Definitions
verb
To satisfy, carry out, bring to completion (an obligation, a requirement, etc.).
Example: You made a promise, son, and now you must fulfill it.
To emotionally or artistically satisfy; to develop one's gifts to the fullest.
Example: This is the most fulfilling work I've ever done.
To obey, follow, comply with (a rule, requirement etc.).
Example: Unfortunately, you don't fulfill the criteria for extra grants at the present time.
To package, distribute, or ship goods.
To fill full; fill to the utmost capacity; fill up.
Example: My lady is positively fulfilled of grace.