Strength of alcohol, and sometimes tannin, that gives an impression of weight and volume in the mouth
When I'm teaching a group of newcomers about wine, one of the most troublesome concepts to get across is weight. Wine is complicated, and sorting out its many sensory factors takes a bit of work. One way I handle the weight issue is to present a range of wines -- from the very light to quite heavy -- so that the differences in weight are more obvious. An analogy that people often find helpful is to look at the weight of different types of milk. Whole milk is rather heavy because of its cream content, whereas 2% is noticeably lighter, 1% milk is lighter still, and skim milk is the lightest of all.
The same idea can be applied to wines, between different wines as well as beetween wines of similar style. Chardonnay is typically heavier than Sauvignon Blanc; Cabernet is heavier than Dolcetto. Then within a given style of wine, or even an individual class of wine, there are also weight differences. Cabernet is noticeable heavier than Merlot (both are what I call ‘Bold & Aristocratic’ reds), just as Chardonnay is usually heavier than Auxerrois -- a grape that was long mistaken for Chardonnay. Then within the same type of wines there are weight differences. Burgundy gives us a good comparison with its various interpretations of Chardonnay. Chablis is the lightest Chardonnay that Burgundy has to offer. Next step up the weight scale would be Burgundy proper, or Beaujolais Blanc. At the top of the ladder are the great and legendary Chardonnays: Pouilly-Fuissé, Montrachet and the like. Of course if you really want to lay it on, you have to turn to New World Chardonnay, with its extra helping of oak and butter.
Weight is actually a more important concept than you might realize. If you think of wine as a food as much as a beverage, then getting a grip on weight is required. We often think in terms of matching food and wine by flavour and/or aroma, but it's far more important to match weight. Light foods and heavy wines do not mix; nor do light wines and heavy foods. So when pairing wine and food, always look for a wine that is at least as heavy as the food, if not somewhat fuller. That way your wine will never be overpowered at the table.