A pinnacle is the highest point of something, whether that's a physical peak or the peak of achievement. When mountain climbers reach the pinnacle of Mount Everest, they stand at the very top. When an architect designs a cathedral with a tall, pointed tower, that spire is the pinnacle of the building.

The word also describes reaching the greatest level of success or achievement. An Olympic gold medal represents the pinnacle of athletic achievement: years of training, countless competitions, and finally standing at the very top. A scientist who wins the Nobel Prize has reached the pinnacle of her field. When a musician sells out the world's most famous concert halls, they've reached the pinnacle of their career.

Notice that pinnacle suggests something rare and hard to reach. Getting an A on a spelling test is an accomplishment, but reaching the pinnacle of spelling means winning the National Spelling Bee. The word carries a sense of grandeur: pinnacles are those peaks that few people ever reach, the absolute highest points whether in geography, careers, or human achievement.