This document defines the terms and definitions relating to the field of dynamic mixing and agitation. It covers the hydraulic characteristics of mixers and agitators. It is intended to contribute to mutual understanding of the various stakeholders in a mixing or agitation project: manufacturers, users, integrators, inspection agencies... This document applies to mixing and agitation systems where there is at least one dominant liquid phase. It does not apply to: static mixers; kneaders; submersible mixers covered by ISO 21630; aerators; pumps. Annex A lists the definitions by alphabetic order.