Definition - What does Rancio mean?
Rancio is an adjective often used to describe certain wines that have undergone a process of oxidation or have been exposed to heat. The term is synonymous with a pungent, off-odor. It is not a negative trait, but characteristic of these wine styles.
A prime example of rancio is Madeira, which is put through a heat process called estufagem, giving it a signature rancio smell.
WineFrog explains Rancio
Rancio is an adjective most specifically used to describe some fortified wines. Just as certain aged cheeses have a pungent smell due to the process by which they are aged and matured, so do some wines, thus, a pungent aroma is usually indicative of a certain style, tradition and quality of wine.
These rancio wines almost always undergo some sort of oxidation process to reach their final style. Others go through an oxidation and heat process.
Some wines that can be described as rancio are Madeira, Marsala, Tawny Port, and Amontillado Sherry.