Plain English Meaning
How much stuff is packed into a given amount of space. Denser air has more air molecules in the same volume; less dense air has fewer.
Full Definition
The mass of a substance contained in a given volume, typically expressed in pounds per cubic foot or kilograms per cubic meter. In aviation, density most often refers to the mass of air in a given volume, which decreases as altitude, temperature, or humidity increases.
Context Anchor
Seen in aircraft performance, weight and balance, fuel and oil discussions, and maintenance descriptions of materials and fluids.
Derivation
From the Latin densus, meaning 'thick' or 'crowded.' That captures the idea well: dense air is air that is crowded with molecules. Thin air is the opposite — fewer molecules in the same space.