A number of definitions come to mind. In music there is the horn as a brass or wind instrument. The name comes from the early days when they were actually made from animal horns. In sound reproduction a horn is a device (again often shaped somewhat like an animal horn) for focusing and projecting the sound emanating from some audio transducer. Usually a horn is attached to a compression driver, and is used to couple the output of the driver efficiently to a larger area of coverage. But a horn may be used on a loudspeaker as well. And, in fact, some speaker cabinets are known as “horn loaded” because they set the speaker back in a cavity that is used to help control the dispersion and project the audio a great distance (long throw). Horns come in all shapes and sizes depending upon the exact duty they are to perform. Fundamental design philosophies have come and gone over the years. Radial horns have all but been replaced by constant directivity horns in many PA applications due to their greater control over dispersion across wide ranging frequencies. The science of horn design is ongoing and we’ll probably continue to see improvements.