I've selected "quiver" as a medium-difficulty word. It's 6 letters long and contains the less common letter 'q', making it moderately challenging. The word has multiple meanings (a container for arrows or a trembling movement), which adds some complexity. It follows all the rules: it's between 4-10 letters, uses only regular English letters, is a real English word (not a proper noun or acronym), and contains no spaces or special characters.