Erdély, Siebenbürgen and Tara Ungureasca
In most languages, including Hungarian and Romanian, places are named after their characteristics. 'Erdély' is no exception. Transylvania used to have lots of forests (still has some), and 'Erdély' in Hungarian literally means 'wooded area' or 'wooded land'. It makes sense that when Magyar tribes arrived from the Pannon steppes, they named this area as such. The Romanian 'Ardeal' has no meaning at all, and it is only a phonetic adaptation of 'Erdély'. By the way, this phonetic adaptation of name places is a general phenomenon in Transylvania. Each ethnic group adapts the name of the other groups or village names. There are many Hungarian name places in Transylvania which are basically variations of the word 'erdő'=forest, and most of their Romanian counterparts follow the same pattern of adaptation (ex. the Hungarian 'Erdőd' becomes 'Ardud' in Romanian )
In the 12th and 13th cent. the areas in the south and northeast were settled by German colonists called (then and now) Saxons. Siebenbürgen, the German name for Transylvania, derives from the seven principal fortified towns founded there by the Saxons.
And let's take a look at the Romanian 'Transilvania'. It obviously comes from the Latin 'Transylvania'. The original form of this name is Transsylvania, a compound word (trans-sylvania), and this is how it appears first in Hungarian chronicles. 'Transsylvania' probably refers to "the land beyond the forests". The Hungarians named this province this way, because coming from the Hungarian plains into Transylvania they had to cross a huge marsh-forest.
Hungarian kings always referred to this province as 'the land beyond the forest', that is Transsylvania. (The official language of the Hungarian Kingdom was Latin.) The Romanians had never used this name until the 19th century. Ironically, in spite of their supposed Latin origin, the Romanians never ever used Latin as their official language. They used Slavic. So, the Romanian word 'Transilvania' is borrowed from the Hungarian regal documents written in Latin.
Actually, it is wrong when we say that the Romanians do not even have their own name for Transylvania. They have. The original Romanian name for this province is "Tara Ungureasca", which literally means 'the Hungarian land/country'. This is how it is mentioned in all Romanian chronicles (written in Slavic), and this is how it was known by the people. Most of the very old Romanian fairy tales, folk songs and ballads refer to this province as "Tara Ungureasca".
Romanians always try to put forward that Transylvania was forcibly conquered by the Hungarians, and that actually Romanians lived there before. However, no nation or ethnic group gives up on its native land. And they would especially not give up the name of their land. On the contrary, the true native name of the land becomes a symbol of cultural identity and independence. It becomes part of the collective memory of a community, and it is transmitted over generations in spite of the oppression. The Scottish or the Irish never called their land England if they were occupied by the English for centuries. Just look at history, and you will see that this is true for every nation. So, why did the 'conquered Romanians' call this province 'Tara Ungureasca' ('Hungarian Country')? Why didn't they continue to call their land on its native original name? And what was that name anyway? The truth is that there was no native original Romanian name for Transylvania, simply because there were no native Romanians in Transylvania at that time. They moved in later, mostly pushed by other nations from the south (Bulgarians and Turks). That is why the only truly Romanian name for this province is 'Tara Ungureasca' (Hungarian Country), and the other two names, Ardeal and Transilvania, are borrowed from the Hungarians.