A critical phenomenon is behavior near a continuous phase transition where fluctuations extend over a large correlation length and thermodynamic responses can become singular. Near a liquid-gas critical point, for example, the distinction between liquid and vapor diminishes while density fluctuations become unusually large.
Correlation length and response functions often obey power laws in the distance from the transition, characterized by critical exponents. Different microscopic materials can share those exponents when their long-distance behavior belongs to the same universality class. The renormalization group explains this by following effective interactions as the observation scale changes. A finite sample, finite observation time or imperfect tuning cuts off the ideal divergences, so experimental power laws need a stated range of scales.
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