What part of speech is the word since?

The word "since" can be a few different parts of speech, depending on its function in a sentence:

* Preposition: This is the most common use of "since." It introduces a phrase that indicates a point in time or an event that began in the past and continues to the present. For example: "I've been working here since 2010."

* Conjunction: "Since" can also be a conjunction, connecting two clauses where the first clause explains the reason for the second. For example: "Since you are leaving early, I'll take care of the dishes."

* Adverb: In rare cases, "since" can be used as an adverb, meaning "after that time." For example: "The train left since. " (This usage is less common and sounds somewhat archaic.)

So, the part of speech for "since" depends on its context in the sentence.