coming in a series before the first: the zeroth level of energy.
Origin of zeroth
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
denoting a term in a series that precedes the term otherwise regarded as the first term
Word Origin for zeroth
C20: from zero + -th ²
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