Aroma is the smell of the brewed espresso. A great espresso aroma is good enough to "eat." The senses of taste and smell are highly intertwined, and a flavorful espresso made from freshly roasted beans typically releases a delicious aroma when served.
A not-so-good espresso aroma is flat, stale, or smells more like cheap drip coffee. Equally bad is an espresso without a discernable aroma.
A good espresso often derives a lot of its pleasant aromas from its crema, so the two criteria are loosely related.