The name Zion (Tziyon) in the Bible can refer to four different places: The Temple Mount, the city of Jerusalem, the entire Land of Israel, and the City of David, south of Mt. Moriah. 

Let us seek out one common thread that will explain the various meanings.

The 16th-century Maharal of Prague, in his famous commentary on Rashi at the end of Parashat VaYetze, explains that the holiness of the Land of Israel stems from the presence within it of Jerusalem. That is to say, Jerusalem is heart of the Land. And from there we can also note that the heart of Jerusalem is the Temple Mount, the site of the Beit HaMikdash – and that the Dvir (the Holy of Holies), where the Tablets of the Covenant were placed, is the heart of the Beit Hamikdash. 

The Dvir is the primary source of the sanctity of the Holy Temple [see Rav Sh. R. Hirsch's commentary to the Torah, Lev. 4, verses 17, 33 et al]. The word dvir is from the same root as dibbur, which means "speech," and this connects the Dvir with the Ten Dibrot, known as the Ten Commandments. The great esteem and admiration that we have for the bringing of sacrifices in the Temple, and the closeness to G-d that it brings, all stem from our adherence and obedience to the words of the Ten Commandments, which include all 613 mitzvot of the Torah [as the 10th-century R. Saadia Gaon stated; see Rashi to Sh'mot 24,12]. 

The Tablets of the Law on which were written the Ten Commandments are literally the handiwork of G-d, as is written: "…the two Tablets of the Testimony, stone tablets written with the finger of G-d" (Sh'mot 31,18). The Torah also states: "And the tablets are the work of G-d, and the writing [thereon] is the writing of G-d, engraved on the Tablets" (Sh'mot 32,16). 

And those very Tablets, given at Sinai, are in the Ark of the Covenant – which itself is in the Dvir, the Holy of Holies, which itself is the very heart of the Holy Temple. After all, the Dvir is the place where no one may ever enter, except for the High Priest on Yom Kippur – and even then, only for a few minutes each of the four times he enters it that day. 

Based on all this, we can understand why the Prophet Yeshayahu states: "For from Zion will go forth Torah" (2,3). 

And with this inspiration of G-d's Torah in the Holy Temple, the wise men of the Sanhedrin convened in the Chamber of Gazit in the Temple, and issued rulings on all matters of doubt having to do with Jewish Law (see Rambam, Laws of Mamrim, Chapter 1).

The word tziyun, which is the same as Tziyon (Zion) but with the shuruk vowel (oo) instead of a cholam (o), means "sign." For instance, we read in the words of the Prophet Yirmiyahu (31,20), "Hatzivi lakh tziyunim," meaning, "Set signs for yourself." Tziyun is also used to mean "gravestone marker" and the like. 

This is the mobile tziyun – but the Tziyon that is quite permanently set in the ground, what we call Zion, is Jerusalem and the Land of Israel. One hundred and fifty mentions of Tziyon in the Bible refer to the Holy Temple, Jerusalem, and the Land of Israel. 

Only in four places does the Bible use the word Tziyon to refer the City of David, which is the area south of the Temple Mount, including the Shilo'ach spring (Nechemiah 3,15). But these four are actually only two, because the verse in Shmuel II 5,7 is repeated word-for-word in Chronicles I 1,5, and the same for Kings I 8,1 and Chronicles II 5,2. The latter pair of verses speaks of "bringing the Ark up from the City of David, which is Tziyon." Why was it called Tziyon? Because the Tablets of the Covenant were there!

But we must ask: Why, in Shmuel II 5,7, was the Fortress of David given the name "City of David" name even before David conquered it? The Metzudot David commentary explains that this is because later, after the conquest, it was given that name. I would like to expand on this and say that it was similarly called "Fortress of David" at an early stage because of the future, i.e., that David was to bring the Ark and the Tablets to that spot. As follows: 

The City of David was called Tziyon in the above verses simply because of the few years in which David placed the Ark of the Covenant, with the holy Tablets inside, in that place. Both sets of Tablets were there: the second set, as written in D'varim, and the first set, those that Moshe shattered during the Sin of the Golden Calf. In addition, an entire Torah scroll was placed in the Ark (Bava Batra 14b).

David was king in Jerusalem for 33 years, after which his son Shlomo took over. From then, it took another seven years for the Temple to be built – and for at least some of this time, the Ark with the Tablets was in the City of David, that is, Tziyon. (When the Philistines returned the Holy Ark to Israel, it was housed in the house of Oved-Edom, from where David brought it to the City of David.)

In short: The word Tziyon signifies the Tablets of the Covenant, which are themselves the sign – tziyun – of the Torah that G-d gave and implanted within us.  

The following objection might be raised: "There was no Ark at all in the Second Temple?!" The answer is that in fact, there was an Ark then, but it was hidden and buried under the ground (Yoma 54a). And since it, too, will one day be restored to us, the name of the site is still "Tziyon." 

And for this reason, the city of Jerusalem, too, and also the entirety of the Land of Israel are called Tziyon – and all in the merit of the Torah that is there!

Translated by Hillel Fendel.