What does "ないしは"mean?
 

Could you provide an example sentence or some added context? Thank you.

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

Also,　"ないし" is sometimes used to indicate numeric range.

株価は1200円ないし1800円の間で上下している。

I think "ないし" means "among the things within a range". When it is applied to two objects, it means "or".