וְאִשָּׁה֙ כִּֽי־תִהְיֶ֣ה זָבָ֔ה דָּ֛ם יִהְיֶ֥ה זֹבָ֖הּ בִּבְשָׂרָ֑הּ שִׁבְעַ֤ת יָמִים֙ תִּהְיֶ֣ה בְנִדָּתָ֔הּ וְכׇל־הַנֹּגֵ֥עַ בָּ֖הּ יִטְמָ֥א עַד־הָעָֽרֶב׃
When a woman has a discharge, her discharge being blood from her body, she shall remain in her menstrual separation seven days; whoever touches her shall be impure until evening.
(The above rendering comes from the RJPS translation, an adaptation of the NJPS translation.)
In this passage on ritual impurity, the force of the noun נִדָּה (which occurs also in vv. 20, 24, 26) is ambiguous. Scholars have put forward three views as to its meaning. One is that נִדָּה is a type of ritual impurity, following a plausible view that this word means something to be shunned. So Ibn Ezra; BDB (lexicon), based on cognates; Susan Niditch (pers. comm.). A counterexample might be the phrase נִדַּת טֻמְאָתָהּ lit. “the niddah of her impurity” in 18:19, which implies that נִדָּה alone means something other than “impurity.” But perhaps the text there is combining two synonyms as an intensifier.
According to other opinions, the term is more emotionally neutral, referring to avoiding contact. So Saadiah at 20:21; Rashi; Rashbam at 12:2 [cf. at 15:19]; Everett Fox, The Five Books of Moses (1995), citing “Buber-Rosenzweig, Greenstein, and others”; Carol Meyers (pers. comm.), noting that “the roots n-d-d and n-d-h [see n-d-h in the piel] are almost synonymous.”
And still others point to a different root and hold that נִדָּה refers to the flow of blood itself. So Baruch Levine (JPS Torah Commentary), ad loc., citing an Akkadian cognate; he understands the root as meaning “to cast, hurl, throw”; Robert Alter.
Here the text does not seem to imply that this source of impurity is any more to be “shunned” than other sources of impurity, such as a penile discharge. This argues against the first view. Of the three options, the second proposed construal of נִדָּה seems the most germane. Although in my view the third proposed meaning does not literally fit in 15:19 and 15:24–25, it would be typical of the Torah to play on it in this passage nevertheless. Therefore it should be taken into account. Usage of the word thus denotes separation while specifying the type of separation that relates to blood flow.
As for translation into English, the NJPS rendering “her impurity” seems to follow the first view above, which misses the mark. The revised rendering expresses the second opinion above, with a bit of help from the third option.