a hole dug or drilled downward, as in a mine or a petroleum or gas well.
occurring or situated in such a hole.
Origin of downhole
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
(in the oil industry) denoting any piece of equipment that is used in the well itself
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