Short definition
A wine named after and made primarily from a single grape variety, like Chardonnay or Merlot.
When a bottle says "Chardonnay" or "Pinot Noir," that is varietal labeling. In the US, a wine must contain at least 75% of the named grape. In Europe, most wines are labeled by place instead — you will not find "Pinot Noir" on a Burgundy label, even though that is exactly what is inside. Knowing common varietals helps you decode both systems.