A pinnacle is the highest or most impressive point of something.
Think about mountains - the very top of a mountain is its pinnacle. It's the highest and often the most challenging part to reach. But pinnacles aren't just about geography. You might hear someone say that their career has reached its pinnacle, meaning they've achieved the greatest success or recognition possible in their field. This could be the peak of their professional life.
How common is "pinnacle"?
Word pinnacle is considered rare in modern English. It has a balanced usage among all categories: speech, fiction, newspapers and academic texts.
Definitions
noun
The highest point.
(geology) A tall, sharp and craggy rock or mountain.
(figuratively) An all-time high; a point of greatest achievement or success.
(architecture) An upright member, generally ending in a small spire, used to finish a buttress, to constitute a part in a proportion, as where pinnacles flank a gable or spire.