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Solid, liquid, and vapor are the three fundamental macroscopic phases of matter used to describe the state of a material under given temperature and pressure. In a solid, particles are arranged in a rigid lattice with limited movement, producing definite shape and volume. In a liquid, particles are less ordered, can flow, and take the shape of their container while maintaining a near-constant volume. A vapor refers to the gaseous state of a material, typically at temperatures where the substance would not be a gas at standard conditions; vapors expand to fill available space and have relatively low density.

Phase transitions describe how a substance changes between these states. Melting converts a solid to a liquid; freezing reverses the process. Vaporization includes evaporation and boiling, transforming a liquid to a vapor, while condensation returns it to a liquid. Sublimation changes a solid directly to a vapor, and deposition converts vapor back to a solid. During most of these transitions, the temperature remains constant while heat is added or removed, corresponding to latent heat of fusion, vaporization, or sublimation.

Phase diagrams use pressure and temperature axes to show which phase is stable. For water, the solid–liquid boundary has a negative slope, reflecting that ice can melt under higher pressures. The water system passes through a triple point at 0.01°C and 611.657 pascals, where solid, liquid, and vapor coexist, and a critical point at 374°C and 22.064 megapascals, where the liquid and vapor phases become indistinguishable.

Understanding solid, liquid, and vapor phases is essential in physics and engineering, influencing everything from material processing and cryogenics to meteorology and daily phenomena such as boiling water and frost formation.