Rheid
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For rhea-like birds (rheids), see Rheidae. For places in Schleswig-Holstein in Germany, see Rheide (disambiguation).
In geology, a rheid /ˈriːɪd/ is a substance whose temperature is below its melting point and whose deformation by viscous flow during the time of observation is at least three orders of magnitude (1,000 ×) greater than the elastic deformation under the given conditions. A material is a rheid by virtue of the time of observation. The term, coined by S. Warren Carey in 1953, has the same Greek root as rheology, the science of viscoelasticity and nonlinear flow.