* Noun: This is the most common usage, referring to a curved line, especially a part of a circle.
* Example: "The rainbow formed a beautiful arc across the sky."
* Verb: It can also be used as a verb meaning "to form an arc."
* Example: "The gymnast arced her body gracefully."
* Adjective: While less common, it can be used as an adjective to describe something that is shaped like an arc.
* Example: "The building featured an arc-shaped window."
So, the part of speech depends on how the word is used in a sentence.