The word coauthor is a noun. The plural noun is coauthors.
Dave and Larry wrote a book together, Dave is the main author and Larry is the coauthor.
It means that they are sharing credit. There is more than one author.
coauthor
no.
It means that more than one author wrote a book/article together.
According to the sources I've found on a quick search, you should hyphenate 'coauthor', i.e., it would better be: "co-author".
Generally co-author should be hyphenated, but the relaxed rules of modern times often present coauthor as one word
The part of speech for this particular word is a noun.
part of speech
The part of speech for "answer" is a noun.
adverb
what part of speech is beneath