Final answer:
In the context of the sentence, 'atmosphere' refers to the layer of gases surrounding a planet, which scatters light and makes the sky appear blue.
Step-by-step explanation:
The student is asking about the meaning of the word 'atmosphere' in the context of the given sentence.
In the sentence 'Because there's not atmosphere, the sky always appears black,' the term 'atmosphere' refers to the layer of gases that surround a planet or celestial body.
Without an atmosphere, there would be no scattering of sunlight, and as a result, the sky would appear black, even during the day, analogous to how the sky looks from space.
This is because the atmosphere causes the 'blue sky' we see due to the scattering of light and is also responsible for various weather phenomena.