Ain't is most certainly a word, albeit one that is used primarily outside of formal writing and speech. It has been with us at least since the 17th century, originally as a contraction for "am not", as an't. It is now used to also mean "is not", with about the same frequency.
The reason it is considered "less of a word" than other contractions really has less to do with its formal structure, and more to do with a series of concerted attacks that began at the beginning in the 19th century - when most of these absurd grammatical prohibitions arose. It quickly fell out of vogue among the "well-educated" classes, and so became a mark of a lower-class speech.
The argument used by most prescriptivists against "ain't" is that its constituent parts are not obvious, and therefore it violates some non-existent rule of contractions - that the apostrophe must replace a literal letter or series of letters. Whether feigning ignorance or under the burden of actual ignorance, prescriptivists from the 19th century on have failed to acknowledge its well-recorded history as a contraction of "am not" and "is not", dating back more than a hundred years before the first objections against the word were raised.
One wonders whether these same prescriptivists consider "won't" a word or not, as it bears just about as much connection to "will not" as "ain't" does to "am not" - arguably more so.
Hopefully that answers your question! Next time someone tells you ain't ain't a word, tell them it has been for more than three-hundred years, and why should it be any less of a word than won't!