First, a word about how my decision trees work. They reflect how I use these words, and that's not always the same as the way dictionaries define them. Often a dictionary will say two words mean the same thing - if I've made a distinction, I'm not disagreeing with the dictionary, I'm just saying that's how I've decided to use the words in question. Language being a living thing and all of that. I mention this because whenever I do one of these, someone (usually my very clever mother) will comment about how I'm incorrect. Please feel free to post about things I ought to've included or how your personal decision tree would be a little different, but understand I'm not trying to map English usage, I'm only mapping my own twisted subdialect of it. End Public Service Announcement.
On with the tree:
So, yes, I know every piece of available information says that a Dirigible is any steerable lighter-than-air vehicle and that it's identical with Airship, but I'll only ever apply the term to Zeppelins and Blimps, never to those damnable thermal airships.
12 hours ago