Integrating earthquake studies with geodynamics requires knowledge of different modes of permanent deformation of rocks beyond seismic failure. However, upon stepping out of the realm of brittle failure, students find themselves in a zone of terminology conflict. Rocks below the brittle shallow part of the lithosphere are said to be ductile, plastic, or viscous, yet in many papers what is obviously brittle deformation is said to be plastic. In this EduQuakes article, I explain the origin of this conflict and how to handle it. The primary reason for the conflict is that the word plastic is used by one research community to describe viscous deformation but by another community to describe permanent deformation that is not viscous. To the former community, emphasis is on microscopic deformation mechanisms. To the latter community, emphasis is on whether the macroscopic deformation is time dependent. Using a Coulomb continuum to approximate the effects of numerous brittle faults adds another level of complexity. It is futile to expect a unification of terminology any time soon, but with some basic knowledge one can live with this situation without suffering scientific confusion.

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