a network of fine lines, wires, or the like placed in the focus of the eyepiece of an optical instrument.
Origin of reticle
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
less commonly reticule
a network of fine lines, wires, etc, placed in the focal plane of an optical instrument to assist measurement of the size or position of objects under observationAlso called: graticule
Word Origin for reticle
C17: from Latin rēticulum a little net, from rēte net
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
1650s, from Latin reticulum "little net," diminutive of rete "net" (see reticulate (adj.)).
Online Etymology Dictionary, © 2010 Douglas Harper