Definition - What does Rotten Egg mean?
In the context of wine making, the term rotten egg is a descriptive adjective for the aroma of hydrogen sulfide and is an indicator that the wine has spoiled. When a bottled wine smells like rotten eggs, the aroma is from hydrogen sulfide and the wine should be discarded. When rotten eggs aroma is detected during winemaking, winemakers can sometimes remedy the situation and save the wine.
WineFrog explains Rotten Egg
Vintners have battled rotten egg taint since they began making wine, as hydrogen sulfide is a naturally occurring compound and can be found on grape skins that have powdery mildew, in some yeast strains and in grapes that have a low nitrogen level. Once the hydrogen sulfide is in the wine, it will begin reacting with the other compounds in the wine and is very hard to control and remove.
Typically, the rotten eggs smell will be detected during the end of fermentation, and if caught in time, the wine can be salvaged by adding copper sulfite and by oxygenating the wine. Popularly, people add a copper penny to bottled wines with an aroma of rotten eggs to mitigate the hydrogen sulfide, but there is only anecdotal evidence to show that this actually works.