A harmonica is a small flat instrument you hold in one hand and put against your lips. You blow air out or suck air in, and you slide it left and right along your mouth to reach different notes. It fits in a pocket, which is a large part of why it spread the way it did — it travelled with soldiers, farm workers and railway workers, and it is the sound people think of for American blues, country music and old folk songs. Two other names for the same thing: 'mouth organ' (older, still common in Britain and among older speakers) and 'blues harp' or just 'harp' among players themselves — so if a musician says 'I play harp', ask which one they mean. There is a curious history under the name: Benjamin Franklin invented a very different instrument made of spinning glass bowls and called it the glass armonica or harmonica, and the little wind instrument borrowed the name later. Almost nobody today means Franklin's instrument. You say 'play the harmonica', the same pattern as 'play the piano'.