Definition
Argon is a colorless, odorless, chemically inert gas that makes up roughly 0.93 percent of Earth's atmosphere by volume, making it the third most abundant atmospheric gas after nitrogen and oxygen.
Plain English
Argon is a small but steady part of the air around us. It does not react with other things, has no color or smell, and is just there as part of the mix we breathe and fly through.
Context Anchor
Seen in basic atmosphere discussions, where the handbook explains what air is made of before covering pressure, temperature, and weather.
Derivation
From the Greek 'argos,' meaning 'lazy' or 'idle.' It was named this because it does not react chemically with other elements -- it just sits in the air doing nothing. That laziness is exactly why it stays a constant background part of the atmosphere.
Intuition Check
Do not think of argon as something pilots normally use or manage in flight. In this context, it is simply one of the minor gases naturally present in the atmosphere.
Example Sentence 1
After nitrogen and oxygen, argon is the next largest component of the air an aircraft flies through.
Example Sentence 2
Because argon does not react chemically, it has no effect on engine combustion or aircraft performance calculations.