Imperative - imperial - imperious
The three adjectives imperative, imperial and imperious are not synonyms, although they share a root in the Latin imperare, 'to command'. An imperator was originally the commander-in-chief, or a general, in the Roman army: after the end of the Roman Republic, it became the favourite epithet of the Emperors, so that its most usual translation into English is 'Emperor' - itself derived from imperator.
- Imperative means 'commanding', or 'necessary', with a suggestion of urgency: "Your mother is ill: it is imperative that you come at once". It has certain technical uses in philosophy; but to users of AWE its most useful application is likely to be in linguistic usage. Here it is most commonly used as a noun. The imperative is one of the moods of the verb: see imperative mood.
- Imperial is the nearest of these three words to its original meaning. It is (or can be) a fairly neutral way of describing 'to do with Empire', and can be found in the studies of History and Politics. For people with some beliefs, it is not very neutral. Imperialists (those who believe in Empire, and, particularly in Britain the British Empire, among people who grew up in it) regard imperialism as a noble and worthwhile belief. Among those who have been subjugated by an Empire, or whose independence is threatened by it, of course, imperialism is a bullying and arrogant creed that must be resisted. Be careful to try to understand the sense in which it is being used in any text in which you find it.
- Imperial measures are the traditional measures used in Britain (and the former Empire) such as yards, feet and inches; gallons, fluid ounces and pints; acres and square miles etc. (There is a fairly full account of this in Wikipedia at [].) The imperial system is in distinction from the metric system, best known as SI (French système international) and US customary units (sometimes called 'imperial units', though it has diverged from the measures current in the UK).
- The prevailing modern sense of Imperious, says OED, is "Overbearing, domineering, dictatorial." It is a word used to describe behaviour and attitude, etc, nearly always at a personal level. It is not a compliment.