Atmosphere
The atmosphere is the layer of gases surrounding Earth, mostly nitrogen and oxygen, held by gravity and divided into layers by temperature.
Definition
The atmosphere is the blanket of gases surrounding Earth, about $78\%$ nitrogen and $21\%$ oxygen with argon and trace gases such as carbon dioxide, held in place by gravity and giving the planet air to breathe, warmth, and protection from harmful solar radiation. It is divided by temperature trends into the troposphere, stratosphere, mesosphere, and thermosphere, with nearly all weather confined to the troposphere and the stratosphere's ozone layer absorbing ultraviolet light, and pressure falls off exponentially with altitude, $P = P_0 e^{-z/H}$, with a scale height of about $8\ \text{km}$. Its present composition is a product of biology, since free oxygen only appeared with the Great Oxidation Event about $2.4$ billion years ago, and its radiative balance today depends on solar input, planetary albedo, and greenhouse absorption by water vapor, $CO_2$, methane, and ozone.
Example
About half the atmosphere's mass lies below $5.5\ \text{km}$, roughly the height of Mount Kilimanjaro, which is why climbers on Everest need supplemental oxygen, and commercial jets cruise near the top of the troposphere at about $10$ to $12\ \text{km}$. Without any greenhouse gases Earth's effective radiating temperature would be about $-18^\circ C$, yet the actual global surface average is about $15^\circ C$, a $33^\circ C$ natural greenhouse effect, and the Karman line at $100\ \text{km}$ is the conventional edge of space even though the thin thermosphere extends much higher.
Key Insight
The atmosphere has no sharp top, just thinner and thinner air fading gradually into space, and if Earth were the size of a basketball the breathable layer would be thinner than a sheet of paper. It is also not in chemical equilibrium: oxygen and methane coexist only because living organisms keep producing both, which is why an atmosphere like ours counts as a biosignature that astronomers now search for on planets around other stars.