What is the term used for food that smells or tastes unpleasant due to being old or exposed to oxygen?

The term for food that smells or tastes unpleasant due to being old or exposed to oxygen is "rancid." This condition occurs primarily in fats and oils when they undergo oxidative deterioration. When fats are exposed to oxygen over time, they can develop off-flavors and odors that are indicative of rancidity. This process not only affects the taste and smell, making the food undesirable, but it can also reduce the nutritional quality of the fats.

In distinguishing "rancid" from other terms, "fermented" generally refers to controlled microbial processes that can produce distinct flavors and are often desirable in foods such as yogurt and pickles. "Stale" typically describes bread or baked goods that have lost moisture and freshness but may not necessarily have developed off-putting odors. "Decomposed" refers to the breakdown of organic matter often due to microbial action, leading to more severe spoilage and an unappealing state, but it is more commonly associated with overall spoilage rather than just the rancid nature of fats.