Aroma-Marriam Webster defines it as: a distinctive pervasive and usually pleasant or savory smell.
In tea terms, this definition also applies. What dictates the aroma of teas? It's easy to figure out each tea has different aroma even from the same region, with same processing and such. In a rough summation, there are several way to determin aromas in tea terms.
1) Varietal aroma - essential aroma of a particular varietal
These varietals posses the essential oils which made them distinctive without any alteration. The aroma may subside but can not be lost. ie Ginger Flower Dan Cong, Yu Lan Xiang Dan Cong.
These are the most unique and should not be altered. They are also the most difficult to make.
2) Charcoal aroma - fire wood smell being smoked onto tea
This is fairly common, Lapsang, Russian Caravan, and some pu-erh, green etc..
3) Scented floral aroma - essential aroma of flowers being obsorbed by tea
This is also fairly popular. Jasmine ring a bell?!
3) Production aroma - natural process of tea to produce/simulate aroma.
Let's use an example: Honey Orchid Dan Cong. There are so many Honey Orchid Dan Cong's out there, how can one tell it's the original aroma or produced to simulate. Techically speaking, they are not the same. The Honey Orchid old bush is essentially aromatic as its name suggested. The commercial production on the other hand is fermented and roasted to tasted like roasted honey with higher fire and fermentation. The results will reveal over time. If both teas were left unsealed for 9 months an on, the former will still have the same aroma, verus the later will loose most of its aroma as the roasting subsides, or become something different.
Roasting can do numbers to the aroma and flavor of a tea. High fire roasting usually means sweeter, fruitier and darker teas, due to the sugar transformation from high temperature roasting. Aroma is significantly changed after high fire. Low fire and long time roasting can mellow out the texture, in turn a smoother cup, aroma is less modified.