1. A female that is connected with a specified thing: testatrix.
2. A geometric point, line, or surface: directrix.
[Middle English, from Latin -trīx, feminine of -tor, n. suff.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
suffix forming nouns
indicating a feminine agent, corresponding to nouns ending in -tor: executrix.
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
a suffix occurring in loanwords from Latin, where it formed feminine nouns or adjectives corresponding to agent nouns ending in -tor; on this model, -trix is used in English to form feminine nouns (aviatrix; executrix) and geometrical terms denoting straight lines (directrix).Also, -trice.
[< Latin -trīx, s. -trīc-]
usage: Most English nouns in -trix have dropped from general use and occur rarely or not at all in present-day English. The forms in -tor are applied to both men and women. When relevant, sex is specified with the generic term: Amelia Earhart was a pioneer woman aviator. Some terms remain in usually jocular use, e.g. editrix. The word dominatrix has connotations that cannot be otherwise expressed. Legal documents still use administratrix, executrix, inheritrix, and the like, but these forms too are giving way to the -tor forms. See also -enne, -ess, -ette.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.