Definition
A nebula is an interstellar cloud of gas and dust. The word once referred to any fuzzy patch in the sky, but today it specifically means these clouds of gas and dust within a galaxy.
In context
Some nebulae are dense regions where gravity is pulling gas together to form new stars. Others, such as planetary nebulae and supernova remnants, are material shed by stars near the end of their lives.
Related ideas
Nebulae glow when nearby hot stars light up their gas, reflect starlight, or block the light behind them as dark silhouettes. The material in them is eventually recycled into new generations of stars and planets.