What is the part of speech word neither?

"Neither" can be two different parts of speech depending on its function in the sentence:

1. Determiner: When "neither" is used to specify that something does not apply to either of two things, it functions as a determiner.

*Example:* "Neither dog is friendly."

2. Conjunction: When "neither" is used to introduce the second part of a correlative conjunction, it functions as a conjunction. The correlative conjunction is "neither... nor"

*Example:* "I like neither cats nor dogs."

So, it's important to look at the context of the sentence to determine the part of speech of "neither."