Deposition (Phase Change)
Deposition is the phase change from gas directly to solid without becoming a liquid first, the reverse of sublimation; frost forming from vapor is an example.
Definition
Deposition is the phase change from gas directly to solid, skipping the liquid state, that occurs when gas particles lose enough energy to lock straight into a solid structure, essentially sublimation running in reverse, and it releases the same amount of energy that sublimation absorbs. It happens when a vapor meets a surface colder than the substance's freezing point, or at low pressure where the liquid phase cannot exist, and formally it occurs when a vapor below the triple point in temperature and pressure becomes supersaturated relative to the solid, so the solid phase has the lower free energy and forms directly; like other phase changes it requires nucleation, typically beginning on surfaces or particles, and in materials science, vapor deposition builds thin films atom by atom this way, while in meteorology it drives snow crystal growth.
Example
On a cold, clear night, water vapor in the air turns directly into frost on car windows and grass without any dew forming first, and snowflakes themselves are made by deposition, not by freezing raindrops, as water vapor high in a cold cloud grows directly into ice crystals. In a cloud at $-15^\circ\text{C}$, the vapor pressure over supercooled liquid droplets is higher than over ice, so water vapor deposits preferentially onto ice crystals, which grow at the droplets' expense, and chemical vapor deposition grows synthetic diamond by decomposing methane in hydrogen plasma so carbon deposits onto a heated substrate.
Key Insight
Frost and dew are genuinely different processes: dew is condensation, gas to liquid, on a cool night, while frost is deposition, gas to solid, on a freezing one, and if dew forms first and then freezes that is frozen dew, not true frost. A snowflake's hexagonal symmetry comes from the water molecule's bond angle and the hydrogen-bonding geometry of ice, and each flake's unique pattern records the exact humidity and temperature it passed through as vapor deposited onto it layer by layer.