What part of speech?

The sentence is "This famous relay service used horses". What part of speech would relay be? On the internet i found that is a noun but it seems like an adjective.

1 Answer

The word "relay" functions as a noun and a verb.
In that sentence, the word "relay" is functioning as a noun called an attributive noun (also called a noun adjunct).

Explanation:

You are correct that "relay" is functioning as an adjective.
An attributive noun is a noun functioning as an adjective to describe another noun (service).