I've chosen "quantum" as a medium difficulty word. It's 7 letters long, which falls within the required range. It has an uncommon letter 'q' paired with 'u', which makes it challenging, but also contains common vowels 'a' and 'u' to give the opponent a fair chance. The word is a legitimate English word referring to a discrete quantity of energy or the field of quantum physics. It's not a proper noun, acronym, or made-up word, and contains no spaces, hyphens, or special characters.