What is an "ism?" The etymologists, harking back – via medieval French and Latin – to Greek words that end in "ismos," will say those three letters are a convenient suffix: add them to a noun or adjective, or to the stem of a verb, and, to the delight of the philologists, you add meaning, distinction and nuance. Terror becomes terrorism; global becomes globalism; baptise becomes baptism. For Messrs Reagan, Thatcher and Marx, their "isms" define their political and economic ideologies, but inventive types will add "ism" to all manner of words – a "Bushism," for example, for any of the many malapropisms uttered by America's 43rd president.For those who want to be on top of their "isms," here is an illuminating and witty guide to more than 400.