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true level

noun
  1. an imaginary surface everywhere perpendicular to the plumb line, or line of gravity.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for true level

true level

noun
  1. a hypothetical surface that is perpendicular at every point to the plumb line, such as the mean sea level or geoida still liquid surface is at true level
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012