நிலை மாற்றம் என்பது பொதுவாக ஒரு பொருள் திட, திரவ, வாயு ஆகிய நிலையில் இருந்து ஒன்றில் இருந்து வேறாக மாறுவதையோ அல்லது அரிதான நிகழ்வான பிளாஸ்மா (இயற்பியல்) நிலைமாற்றங்களை விவரிக்க பயன்படுகிறது,

In chemistry, thermodynamics, and other related fields like physics and biology, a phase transition is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states of matter: solid, liquid, and gas, and in rare cases, plasma. A phase of a thermodynamic system and the states of matter have uniform physical properties. During a phase transition of a given medium, certain properties of the medium change as a result of the change of external conditions, such as temperature or pressure. This can be a discontinuous change; for example, a liquid may become gas upon heating to its boiling point, resulting in an abrupt change in volume. The identification of the external conditions at which a transformation occurs defines the phase transition point.