Well, it denotes the process an essential oil went through to be distilled. Ottos are steam distilled, while absolutes, the other common essential oil type, usually employ solvents to extract the oil. Steam distillation is a process that's been around for centuries, and in antiquity, India, Syria, and Persia (now Iran) were known for their gorgeous floral ottos. Ottos also smell a little different than an absolute; because of the heat employed in the process, ottos don't smell like fresh flowers. Rather, they have a heavier, heady quality. You can find otto these days in fragrances like Penhaligon's Hammam Bouquet ($110) and REN Moroccan Rose Otto Bath Oil ($48), or Malin+Goetz's Otto candle ($48).