I've chosen "molecule" as a medium-difficulty word. At 8 letters long, it fits within the required length. It's a scientific term that most people would recognize, but not a word used in everyday conversation, making it moderately challenging. It contains common vowels (o, e, u) and consonants (m, l, c), but the arrangement provides a good challenge. This word follows all the requirements: it's between 4-10 letters, contains only standard English letters, is a real English word (not a proper noun or acronym), and has no spaces or special characters.