And God called the firmament Heaven. So the evening and the morning were the second day.Genesis 1:8 (NKJV)
God (‘ĕlōhîm) called the firmament Heaven.
“Heaven” is translated from the Hebrew word šāmayim. It means heaven, heavens, air, or sky.
This word can also be found in Genesis 2:19:
“Out of the ground the LORD God formed every beast of the field and every bird of the air (šāmayim), and brought them to Adam to see what he would call them. And whatever Adam called each living creature, that was its name.”
It is important to note that evening was listed before morning in sequence here. This is why Jewish days officially begin and end at sundown.