the outer layer of the thermosphere
The boundary between Earth's atmosphere and outer space is generally considered to be around 62 miles (100 kilometers) above sea level. This boundary is known as the Kármán line. Anything beyond this point is considered to be in outer space.
Well, its not the "outer" space, but its called the space, as it is the last layer of the atmosphere. It called "space" because its distance above the earths surface is 500km to god knows what !