Vinaya
In the lineage of the vinaya, the requirements for ordination as a monk or a nun include the presence of at least five other monks, one of whom must be a fully ordained preceptor, and another an acharya (teacher). This lineage for ordaining nuns became extinct in many Buddhist countries. When Ani Tenzin Palmo wanted full ordination for example, she had to travel to Hong Kong to receive it.
Read more about this topic: Lineage (Buddhism)