Definition - What does Flat mean?
"Flat" is a term used directly to describe an effervescent wine that has lost its bubbles. For other wines, it is an adjective which can also be interchanged with "flabby." This is used to denote a wine which lacks balance in its structure, particularly in its acidity on the finish.
WineFrog explains Flat
If the adjective "flat" is used to describe any effervescent wine (those which have undergone the traditional méthode champenoise, charmat or other method to produce carbonation) that has lost its effervescence. In another context, the adjective "flat" is sometimes replaced with "flabby" or "fat." These wines are considered to be out of balance and unrefined. They are heavy on the palate, with little to no complexity and lack any presence of acid on the finish.