Question: What does 'thin-air' mean?

Damian Bailey answered on 17 Jun 2013:

Thin-air basically refers to less oxygen in the atmosphere because the barometric pressure is lower. This is what happens when we travel to high-altitude to climb the big mountain giants! The air that surrounds you (you can’t see it!) weighs something (and so it exerts a pressure) and we can measure that with a barometer (helps tell us if it’s going to be a nice day or rainy day). Galileo’s student (Evangelista Torricelli) was one of the first guys to show that air gets thinner at high-altitude (cool!). This explains why we huff and puff when we climb higher and higher because there’s less oxygen available! We even measure pressure in the lab using its unit of measurement (Torr, named after Torricelli; cool!).