The literature dealing with the water conducting properties of sapwood xylem in trees is inconsistent in terminology, symbols and units. This has resulted from confusion in the use of either an analogy to Ohm's law or Darcy's law as the basis for nomenclature. Ohm's law describes movement of electricity through a conductor, whereas Darcy's law describes movement of a fluid (liquid or gas) through a porous medium. However, it is generally not realized that, in their full notation, these laws are mathematically equivalent. Despite this, plant physiologists have failed to agree on a convention for nomenclature. As a result, the study of water movement through sapwood xylem is confusing, especially for scientists entering the field. To improve clarity, we suggest the adoption of a single nomenclature that can be used by all plant physiologists when describing water movement in xylem. Darcy's law is an explicit hydraulic relationship and the basis for established theories that describe three-dimensional saturated and unsaturated flow in porous media. We suggest, therefore, that Darcy's law is the more appropriate theoretical framework on which to base nomenclature describing sapwood hydraulics. Our proposed nomenclature is summarized in a table that describes conventional terms, with their formulae, dimensions, units and symbols; the table also lists the many synonyms found in recent literature that describe the same concepts. Adoption of this proposal will require some changes in the use of terminology, but a common rigorous nomenclature is needed for efficient and clear communication among scientists.