Grammar
Word: השמים or שמים
Representation: HShMYM (H + ShMYM)
Transliteration: hashamayim or shamayim
Noun: (H + ShMYM) "The heavens" or (ShMYM) "heavens"
Strong Concordance: H8064
Exposition
The etymology of "heavens" is interesting, so let's begin with that.
Before the English word there was the Latin Vulgate of "caelum", which is a singular noun. (The Roman sky God was Caelus, based on this word). In the Vulgate its meaning was "[vault] of the sky/heaven", as in the protective shelter. The Christian fathers who accepted this included such illuminaries as Origen ("without doubt firm and solid" - First Homily on Genesis. FC71), Ambrose ("the specific solidity...is meant" - comment on Genesis 1:6 in Hexameron, FC 42.60), and Augustine ("indicate[s] not that it is motionless, but that it is solid" - The Literal Meaning of Genesis. ACW 41.1.61)
Before the Latin word there was the Greek Septuagint which used "οὐρανὸν" (ouranos) which is also a singular noun. It's original meaning was "vaulted expanse". Ouranos was also the name of the Greek God of the sky. The Greeks, like the Jews, also believed in a solid dome overhead. Greeks from Anaximens to Aristotle set it forth a fact that it was solid, and Ptolemy's Almagest explains how the stars were fixed to its underside, like nails.
And before all of these, we have the Hebrew word, which is always in plural and is but a renaming of the overhead protection from the waters above. (see Genesis 1:8 when "rakia" is renamed "shamayim"). Rabbinical writings, such as the Talmud (tractate Pesachim and other places), go into detail trying to determine its thickness, among other attributes of shamayim (which they never considered a place where the souls of the dead reside).
If you have not figured it out yet, "firmament" and "heavens" are the same thing.
The Hebrew Word "shamayim"
“Shamayim” (ShMYM) is almost always translated to “the heavens” (the prefixing “H” adds a “the” to the word, so this page will appear for "heaven" or "heavens" since the use of the prefix is inconsistently used).
It should be noted that this Hebrew word, just like “water” (MYM), is always plural. When translating “water” into English, we sometimes use the singular, and sometimes the plural, depending on English grammar usage, and “heavens” should be given this same consideration.
There are anywhere from one to seven heavens, depending on the ideology (perhaps more). In the plain text, there are 2 instances where it is defined, so one can reasonably hold it as either one or two.
When used in Genesis 1:1, it is referring to the object that will be introduced in the following verse: the upper surface of “the deep [waters]” (what will later become “the upper waters”), which is covered by a layer of darkness. And when used in Genesis 1:8, this same word is referring to the raqia or “firmament”, which is pressed against the under-surface of the upper waters, which, supposedly, can be seen from the land below.
In both cases, it identifies a watery upper-surface/under-surface or body of the upper waters
.
There are two important Midrash stories that explain this.
The first is that “shamayim” (ShMYM) is a contraction of two words: “sham” (ShM) plus “mayim” (MYM), meaning “water is there”. In other words, it is a term to explain the surface of the waters appearing in Genesis 1:2, and the under-surface in verse 1:8 that will be seen from the land below when it is revealed later on and renamed.
The other Midrash is that “shamayim” is a contraction of “aish” (AISh) and “mayim” (MYM), or fire and water. It was held by many, such as the Ramban, that the darkness that lay on the water was a fiery darkness, and so, from this point, “shamayim” is speaking only of verse 1:2, and verse 1:8 is unrelated at all.
Either position works.
It is important to remember that actual “water” is the key component of “heavens”. "Heavens", as used in the Pentateuch, is not a concept, nor another word for "atmosphere" nor a supernatural realm, but either represents the upper surface, the combination of the upper surface and the layer of darkness, or the upper
and lower surfaces and the darkness or firmament that presses against them (In verse 1:2, the darkness presses against it, and in verse 1:8, it is the firmament).
And for those who believe in multiple heavens, they can be all three, and more!
Again, we are speaking of physical objects here. I explain more about what the firmament is
here.