WHAT'S a cappuccino? In The Complete Book of Coffee writer Harry Rolnick defines cappuccino as half espresso, half milk and foam. In Foodspell, a reference guide endorsed by the Newspaper Food Editors and Writers Association in the United States, cappuccino (spelled with double p and c) is defined simply as Italian ''coffee with steamed milk''. Ask for cappuccino in Hongkong, and what arrives ranges from hot milk with a splash of coffee to espresso with a whisker of milk foam. The definition may blur. But the fact remains: cappuccino is the number one, best selling coffee drink around. Though Italians sip bracing espresso through a milk foam, local tastes dictate a splash of milk. Or more, much more milk. According to local coffee-makers, a cappuccino is a marriage of rich, nutty espresso, varying amounts of steamed milk and a heap of thick, stiff milk foam. Add a dusting of (in their order of popularity) chocolate, cinnamon or nutmeg, and you've got a relatively skinny substitute for a rich dessert. A cappuccino ranges from $12 at a carry-out shop to $35 in the lobby of a five-star hotel. ''Many customers think it is dessert,'' said Karen Wang, general manager of Cafe Afrikan. ''When it arrives, they think the foam is actually whipped cream.'' At Grappa's in Pacific Place, cappuccino sells for $22 with the preferred topping, chocolate. ''It's more popular than espresso or normal coffee,'' said manager Sandra Lorenzi. On an average day Grappa's serves 200 cappuccinos versus 100 expresso. ''Diners usually order it after dinner. But, on Saturday and Sunday, people order it all day.'' Approximately 90 per cent of Tom Neir's customers at Pacific Coffee Company in the Bank of America Tower order cappuccino.