Chloroform is a clear, heavy liquid with a sweet smell. If you breathe enough of it, you become unconscious and feel no pain. That is why doctors in the 1800s used it to put people to sleep for operations. Though actually, it is not a kind sleep: too much can stop the heart, so hospitals gave it up long ago. You still meet the word in crime stories, where someone holds a cloth to a face and the person drops at once. Real life is slower and far more dangerous than the films. Chemists still use it in the lab to dissolve other things. You can also use it as a verb: to chloroform someone means to make them unconscious with this liquid. It is a serious, slightly old-fashioned word; everyday hospital talk now says anaesthetic instead.