A phase refers to a distinct state of matter that characterizes a substance under specific conditions of temperature and pressure. The most commonly recognized phases include solid, liquid, gas, and plasma, each exhibiting characteristic bulk physical properties. Phase transitions, such as melting, boiling, or deposition, occur when a substance moves from one phase to another due to changes in external conditions. These transitions are governed by intermolecular forces and the energy relationships between the substance and its environment. Understanding these phases is fundamental to fields ranging from atmospheric science to material engineering, as the observable properties and reactivity of matter are fundamentally dependent upon its current physical state.