Ni l'un(e) ni l'autre ne... means neither one nor the other or neither (of them).
Look at these examples:
J'ai appelé Paul et Hadrien, mais ni l'un ni l'autre n'a répondu.I called Paul and Hadrien, but neither one nor the other answered
L'un/l'une agrees in gender with its antecedent.
Note also that the verb following is conjugated as with il/elle.
Ni l'un/e ni l'autre can also be used on its own to express neither:
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Examples and resources
Quel parfum tu veux ? Fraise ou vanille? - Ni l'un ni l'autre.What flavour do you want ? Strawberry or vanilla? - Neither.
Mes filles sont bonnes en maths, mais ni l'une ni l'autre n'est bonne en français.My daughters are good in maths, but neither [one nor the other] is good in French.