In the English language, there seems to be a term for any word that shows up in a sentence; be it a noun, verb, adjective, etc. What's the correct term for "a" or "an" in a sentence? This is actually much harder to Google than it sounds due to Google stripping out simple words such as "a" and "an".
Those words are known as indefinite articles — "indefinite" because they don't refer to a specific/particular thing, like the definite article, "the", does. They do refer to an individual thing; it's just not any particular one that's been singled out previously.
There are also so-called zero articles. These show up when no word at all is used as an article, such as before "so-called" in the previous sentence, which could have been worded as "There is also a so-called […]". This is usually to indicate a group, rather than an individual. (Both definite and indefinite articles, while usually referring to an individual thing, can be used to refer to an individual characterizing a group, and therefore to the group as a whole by extension.)
"Some" is sometimes considered an article as well, as in "He had some ninety-nine answers."