Achievement
Accomplishment
noun
One achievement plus another achievement is an even bigger achievement. Wow, what an accomplishment!
功績 is a formal word that refers to noteworthy contributions or successes. It's commonly used to praise someone's hard work or great results.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
成功が誰の功績かを判断するのは難しい。
It’s difficult to decide who deserves credit for success.