A biosensor is a sensor that uses a biological component like an enzyme or antibody to detect a single analyte. It consists of a biological recognition element and a physiochemical transducer. Leland Clark invented the first biosensor, the Clark oxygen electrode, which allows real-time monitoring of blood oxygen levels during surgery. Biosensors detect analytes using techniques like fluorescence, DNA microarrays, surface plasmon resonance, and electrochemical methods. They have applications in food analysis, medical diagnosis, environmental monitoring, and more.