It certainly is English, it's right here in my Merriam-Websters Tenth. In fact, it's a great trivia question to ask someone to name a five syllable word that is only three letters long. (Pronouncing it "if-fuh" is also permitted, but I think unusual because it sounds so much like "if" while having a very different meaning.)

It's a pity that "xor" isn't a word, because we need that too. "and/or" is an abomination.