Mount Everest

Short definition from Wiktionary

Proper noun

1. 1
   A mountain in the Himalayas, on the border of Solukhumbu district, Koshi, Nepal and Tingri County, Shigatse, Tibet Autonomous Region, China; the world's highest mountain above sea level.

Noun

1. 1
   An endeavor that is very demanding yet rewarding; also, something which is the highest achievement, challenge, etc.; the epitome, the pinnacle, the ultimate.