[tomohiro]

it can be translated as "or".
This is an example from Yahoo news:
株式市場参加者、ないしは債券市場参加者のどちらかが 間違っている

Im not good at English to translate the sentence, but the author of the article used this form,
A, ないしはBが間違っている。＝A or B are mistaken

This news article is about financing. I think the word ないし is often seen in formal writing because people don't casually use it.
hope this will help you