means A young stellar object, typically less than 10 million years old, surrounded by a disk of gas and dust, characterized by violent outbursts and erratic brightness changes as it contracts toward the main sequence.

from Named after T Tauri, a star in the constellation Taurus discovered in 1852 to behave oddly—dimming and brightening unpredictably. Astronomers realized in the 1940s that T Tauri wasn't unique; it was simply young, and its instability was the signature of stellar adolescence. Now T Tauri is the prototype for an entire class of pre-main-sequence stars.