WINE HAS a vernacular all its own, used by experts, amateurs and enthusiasts to convey a sense of what they are testing--and of what they know.

Thus, while much of what is said about wine is fanciful rather than factual, there is some common jargon one should know to understand what all the wine experts are really saying over that glass of chardonnay or cabernet sauvignon. Knowing these terms will help even the unknowing feel more at home in just about any wine circle:

ACETIC ACID--When the finely tuned wine nose proclaims the wine to possess excessive acetic acidity, chances are the wine is in some state of disintegration. Acetic acid gives wine the smell of vinegar. It is often a result of a defective winemaking process. Acetic acidity is caused by bacteria that must have air to develop their damaging characteristics.

ACIDITY--All wines contain natural fruit acids, most commonly tartaric, lactic, citric and malic. Acidity preserves the wine's freshness and fruity character. Wines made from the unripe grapes of a poor vintage are often too high in acidity. Conversely, if grapes are left too long on the vine in a hot climate, they become overripe and lose their acidity. Acidity is usually expressed by a winemaker as total acidity, and the best made wines have between .6 percent and .7 percent.

AROMA--The smell of a new wine prior to bottling.

ASTRINGENT--A wine said to be astringent is usually a young red wine, sometimes a white wine, that has a lot of tannin, a mouth-puckering substance produced by grape skins, seeds and stems.

BARREL AGING--The process of putting wine into relatively small, new or old oak barrels to impart to the wine a slight taste of oak. It is a characteristic deemed complex by connoisseurs. Most big red wines such as top cabernet sauvignons and pinot noirs and many fine chardonnays are aged in oak barrels from as little as a month for white wines to three or more years for red wines.

BARREL FERMENTATION--A process in which the sugar in the grape is converted to alcohol inside small, 55-gallon barrels rather than massive oak, concrete or metal vats. Most winemakers who barrel-ferment their wines are aiming for maximum quality. The rewards are many, but the risks and expensive labor involved often preclude this step by a majority of wineries.

BODY--The sum total of the weight of all the wines' components, alcohol, fruit extract, glycerine and sugar. Wines can be light bodied, medium bodied or full bodied.

BOUQUET--The concoction--sometimes marvelous, sometimes insipid and foul--of smells and scents that the wine develops in the bottle as it ages. As the wine ages and matures, the bouquet continues to develop extra dimensions until the wine passes its prime.

BRIX--The term used by winemakers to refer to the percentage of sugar in the grapes at the time of the harvest. A wine with a brix of 24 degrees, the norm for many wines in California, has almost 1/4 percent total sugar in its composition.

CHAPTALIZATION--A wine that has been chaptalized--a process largely impossible to detect if properly done--has had unrefined beet or cane sugar added to the grape must to increase the alcohol content. This practice, prohibited in California, is permitted in parts of France when the vintage does not produce fully ripe grapes.

FAT--Wines that are described as fat frequently share a rich, fruity, full-bodied character with plenty of flavor and depth, but suffer from below-average acidity as a general rule.

FILTERED--A process employed by the winemaker to remove suspended particles existing in the wine. Filtration can vary from a light filtration to create a brilliant, clean wine, to a sterile filtration, which removes from the wine much of its life-giving components; with such extreme filtration no sediment will form in the bottle.

FINING--A traditional practice used to clarify wines, fining involves introducing the whites of eggs or, more commonly today, powdered clay (bentonite) or gelatin to the wine. These materials act as a magnet for suspended particles, attracting them into a mass, which falls to the bottom of the barrel or tank and is removed.

MADERIZATION--A wine said to be maderized is usually brownish in color with a weak, faded, past-its-prime flavor.

MALOLACTIC FERMENTATION--Many wines go through two fermentations. The first fermentation, wherein sugar is converted to alcohol, is called the primary fermentation. Some wines, particularly red wines and white wines naturally high in acidity, go through a second fermentation called malolactic. In this particular fermentation the tart malic acid is converted to softer lactic acid. Most fine red wines go through both fermentations whereas many white wines never go through a malolactic fermentation.

MERCAPTAN--This is an odor, and certainly a wine flaw, that is a result of a vineyard's spraying sulfur dust to protect against mildew, and then picking the grapes too soon after spraying. The new wine is characterized by the stench of rotten eggs. Once present in a bottled wine, it will not go away and the wine is hopelessly spoiled.

NOSE--The scent, bouquet and aroma given by a wine is referred to as its nose.

OXIDATION--Wine's most constant enemy is prolonged exposure to the air. Wines exposed to too much air during either the fermentation or aging process take on a stale, dumb, unpleasant flavor and aroma. A wine that is oxidized is obviously flawed.

PH--A chemical term that expresses a measurement of the acidity in a wine (technically a measurement of hydrogen ions). The normal Ph range for well-balanced wines is 2.9 to 3.6. A Ph of 2.8 or lower indicates a sour, tart wine, while a Ph above a 3.6 indicates a fat, flat, overly acid-deficient bottle of wine.

RACKING--A very traditional process of pumping wine from one barrel to a clean barrel, to remove the wine from a deposit it has shed. A very costly labor-intensive process, racking helps clarify a wine and prevents the wine from becoming too harsh as a result of sitting too long on its deposit, or "less."

RESIDUAL SUGAR--All dry wines are usually fermented so as to convert all natural sugar to alcohol. Some winemakers desire a bit of residual sweetness in the wine, and therefore will arrest the fermentation. Sometimes the fermentation stops naturally before all the sugar converts to alcohol. Residual sugar is usually expressed as a percentage of the total wine, and very sweet dessert wines can easily have between 15 percent and 25 percent remaining sugar in the wine.

SEDIMENT--The deposit that a fine wine precipitates after several years in the bottle. The appearance of sediment is a healthy sign: it signifies that the wine has not been overly filtered or processed too much. Wines with sediment should be decanted to maintain their clear brilliance.

SUPPLE--A descriptive term used to indicate that a wine, while having good structure and balance, also has a pleasing, tactile, lush feeling in the mouth. A very desirable trait.

TANNIN--The component of red wine and occasionally of white wine that gives an astringent, puckery character to the wine's taste. It acts as a preservative, and gradually dissipates as the wine ages in the bottle.

TARTRATES--Flavorless, harmless, often whitish crystals that resemble coarsely ground salt, found on the inside of the cork, or as sediment at the bottom of the bottle. They result from a wine being exposed to cold temperatures, causing potassium bitartrate to crystalize. Most wineries now cold-stabilize their wines and then filter out the tartrate crystals that have formed.

VOLATILE ACIDITY--Abbreviated as VA, it is always present in wine; however, it becomes a problem when the level of VA gets too high. VA is a by-product of the fermentation of wine. While even a well-made wine possesses certain amounts of VA, if it exceeds acceptable limits, it gives the wine a vinegary off-smell, which effectively precludes enjoyable drinking.