I'm not sure we have a consensus on what the word spicy means for wine. We hit the same problem with that strange word, smooth. Smooth as a descriptor seems to have teleported in from old cigarette ads, and it's not exactly clear that the word describes. But what smooth lacks in precision, it gains in salesmanship. Not long ago, at a wine tasting, my friend John said to me, "If I owned two vineyards, I'd name one Spicy, and the other Smooth. I'd make ten million dollars." Clearly, John has a mind for marketing.
For spice, the confusion likely stems from the fact that the word refers to three different kinds of experiences, and each has a different origin.
The first, and probably the easiest to recognize in a glass, is the appearance of black or white pepper flavors. These are most associated with wines made from the Syrah grape, especially those from the northern part of France's Rhone valley. In 2008, the Australian Wine Research Institute figured out that the culprit is a chemical compound called Rotundone, which is present in both Syrah and in peppercorns.
The second type of spice is related to the first. Syrah's peppery Rotundone fits into a class of aromatic compounds called terpenes. These compounds create floral scents in plants, and they exist in many wine grapes, too. Riesling, Viognier, and Gewurztraminer are all endowed with their share of terpenes, and the notes can be direct enough that we can call them spicy (the Germans go ahead and commit to the word; würz in Gewürztraminer is the German word for spice).
A third (and probably most common) way we encounter spice in a wine has to do with that smell and flavor of new oak barrels, which impart flavors that remind us of baking spices like cinnamon, clove, and nutmeg. Wines that carry strong smells of oak, like Rioja or certain styles of Zinfandel, are good demonstrations of this effect. And I cannot be sure, but I've heard that they can be smooth, too...
About the Author: Steven Grubbs is a sommelier and wine director at Empire State South (Atlanta, GA) and Five & Ten (Athens, GA). Ask him what to drink on Twitter, where he also accepts questions on tacos and manhood.