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.]
suffix forming nouns
indicating a feminine agent, corresponding to nouns ending in -tor: executrix.
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.