Slow permanent deformation under a sustained load, below the yield strength.
Creep happens because materials flow over time under stress, even below yield. It becomes significant at temperatures above roughly 30% of the melting point in kelvin for metals — around 300 °C for steel, 150 °C for aluminium, and room temperature for many plastics.
It is why a plastic part under sustained load deforms without ever being overloaded, and why PTFE gaskets take a set. Design for it by keeping sustained stress low, or by choosing a material with a higher temperature capability.
Designing a plastic part for a sustained load from short-term tensile data. Creep can double or triple the deformation over weeks.
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