A young low-mass star characterized by variability, the presence of hydrogen emission lines, and association with dark or bright nebulae. T Tauri stars are named after a variable star in the constellation Taurus that exhibits particularly strong hydrogen emission. They were originally believed to be ordinary field stars passing through and interacting with a star-forming nebula. Their association with these regions was soon discovered to be more than coincidental, however, and they are now identified with the earliest phase of young stellar evolution, in which a star emerges from its natal molecular cloud to be detectable at visible wavelengths. T Tauri stars with very strong emission lines are designated classical T Tauri stars; their counterparts with reduced hydrogen emission are known as weak-line T Tauri stars. In addition to the original defining properties, T Tauri stars are generally accompanied by excess x-ray, ultraviolet, infrared, and millimeter-wave emission that arises from an accreting circumstellar disk of dust and gas.