= Critical phenomenon
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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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