It is, but it actually has two different meanings. There is what you mean by it (the traditional military definition) and then there is “assault rifle”=“rifle which was legally classified as an assault weapon under the 1994-2004 US Federal Assault Weapons Ban”-a civilian AR-15 is (generally) not an “assault rifle” in the first sense, but it is in the second. Now, you can argue the second is an abuse of language - but once a usage becomes supported by US federal law (even if since expired), that becomes an ipso facto legitimate definition, and it would be better to acknowledge people who use that definition as doing so than lecture them that they don’t know what they are talking about (maybe they don’t, but when you don’t acknowledge the different definition of the term which they are actually using, it doesn’t make you sound like you do either)
See [0], "a United States federal law which included a prohibition on the manufacture for civilian use of certain semi-automatic firearms that were defined as assault weapons".
"The Act prohibited the manufacture, transfer, or possession of "semiautomatic assault weapons," as defined by the Act."
0: https://en.m.wikipedia.org/wiki/Federal_Assault_Weapons_Ban
But calling an AR-15 a "military assault rifle" is probably enough evidence that you don't know what you're talking about, anyways. ('Assault Rifle' is a term of art and the vast vast majority of AR-15s do not qualify - the legal ones that do qualify have literally never been used in violent crimes in the US, because they are worth tens of thousands of dollars and machine guns (a superset of assault rifles) have had universal background checks since the 1920s)