Answer:
Adverb.
Step-by-step explanation:
An adverb can be defined as a word that is used in English language to modify a verb, adjective, or another adverb. Some examples are slowly, quickly, brightly, sadly, etc.
When an adverb is used to refer to a whole sentence in order to express an opinion, describe an attitude or simply act as a comment, it is known as a sentence adverb. Some examples of this type of adverb are however, clearly, sadly, evidently, fortunately, seriously, curiously, actually, presumably, apparently, etc.
FIRST, we will stay two nights in the cabin.
The word "first" in the sentence above is a sentence adverb. Also, sentence adverbs should be followed by a comma.