* Adverb: This is the most common usage. It modifies a verb, adjective, or another adverb to express intensity, emphasis, or disbelief.
* Example: "Are you seriously considering quitting your job?"
* Interjection: Used to express surprise, disbelief, or annoyance.
* Example: "Seriously! I can't believe you forgot my birthday!"
* Adjective: Though less common, "seriously" can be used as an adjective to describe something as serious or important.
* Example: "This is a seriously important issue."
So, to determine the part of speech, you need to consider the context in which it's used.