A sensor is an artificial device that analyzes energy patterns to identify objects and phenomena. Sensors can be passive -- only analyzing energy received from external sources -- or active -- generating energy pulses and analyzing "echoes" that return from them. Modern examples of sensors include RADAR and SONAR, which use radio waves and sound waves, respectively, to detect and identify objects; these sensors can be either active or passive. Examples of modern-day sensors that are passive-only include motion sensors, digital video cameras, and microphones.
Sensors in Science Fiction
Most science fiction spacecraft possess an array of futuristic sensors capable of detecting other space craft at great distances, detecting living things on ships or planets, analyzing the composition of materials, and various other tasks. Many science fiction universes have FTL capable sensors.