1. Adjective: When describing something that is similar or corresponding in some way, like parallel lines.
Example: "The roads run parallel to each other."
2. Noun: When referring to the state of being parallel or to a parallel line or object.
Example: "The parallel between the two stories is striking."
3. Verb: When referring to the act of moving or being in parallel.
Example: "The two trains paralleled each other along the track."
4. Adverb: While less common, "parallel" can function as an adverb, meaning "in a parallel manner."
Example: "The two dancers moved parallel to each other."
So, the part of speech for "parallel" depends on the specific context of the sentence.