Green fluorescent protein (GFP) is a protein originally found in the jellyfish Aequorea victoria. It emits bright green light when exposed to ultraviolet or blue light, making it a valuable tool in biological research. Scientists use GFP as a marker to study cellular processes. By attaching GFP to other proteins, researchers can visualize where and when these proteins are active in living cells, helping to advance our understanding of various biological functions and diseases.