A ditty is a little song or a simple tune. When you're babysitting, you might sing a soft ditty to help a child fall asleep.
A ditty might be a simple nursery rhyme in song form, or even the theme song to your favorite TV show. Ditties are easy to learn, and fairly short and effortless to sing. The word comes from a French word, ditie, "composition, poem, or treatise," from the Latin root dictatum, "thing dictated."