Green fluorescent protein (GFP) is a naturally occurring protein found in the jellyfish Aequorea victoria. It emits bright green fluorescence when exposed to ultraviolet or blue light, making it a valuable tool in biological research. Scientists use GFP as a marker to visualize and track proteins, cells, and other biological processes in living organisms. By attaching GFP to specific proteins, researchers can observe their behavior in real-time, enhancing our understanding of cellular functions and interactions.