plume plume (plo͞om) n. 1. A feather, especially a large and showy one. 2. A large feather, cluster of feathers, or similar ornament worn on a helmet, hat, or horse's harness. 3. A token of honor or achievement. 4. A mass or stream of material that resembles a long feather: a plume of smoke. 5. An area or section of air, water, or soil containing ...