means The highest point of something — a mountain's summit, a graph's apex, or the strongest moment of a trend or career before it levels off or declines.

from From Middle English, related to 'pike' and 'peke' — words for a sharp, pointed thing. The family tree is tangled: 'peak,' 'pike,' and 'beak' all seem to share an old sense of something jutting or pointing. By the time we got mountain peaks and peaked caps, the word had settled into meaning 'the pointy top.' The figurative 'peak of one's powers' came later, once English decided that anything could have a summit, even a Tuesday.