If you ask a botanist…
He or she will tell you that a tomato is a fruit. Anything with a ripened, mature ovary containing a seed or seeds is technically a fruit. By this definition, pumpkins, peppers, and even eggplants are also fruits. If you look "tomato" up in the dictionary or on Dictionary.com, you will probably also find it defined as a fruit.
But, horticulturally speaking…
The tomato plant, itself, is classified as a vegetable. It is an annual as opposed to a perennial woody plant or a tree from where most of the things we consider fruits such as apples, cherries, and oranges come.
And, according to the federal government…
The tomato also seems to be considered a vegetable. In 1893 the U.S. Supreme Court actually ruled in the case of Nix v. Hedden that the tomato is a vegetable (at least, that is, within the meaning of the Tariff Act of 1883.) You see tariffs had been placed on vegetables, and domestic growers of tomatoes wanted the same import taxes to apply to tomatoes that applied to other vegetables coming into the country.
The Supreme Court agreed, and Justice Gray declared, "Botanically speaking, tomatoes are the fruit of a vine, just as are cucumbers, squashes, beans, and peas. But in the common language of the people, whether sellers or consumers of provisions, all these are vegetables which are grown in kitchen gardens, and which, whether eaten cooked or raw, are, like potatoes, carrots, parsnips, turnips, beets, cauliflower, cabbage, celery, and lettuce, usually served at dinner in, with, or after the soup, fish, or meats which constitute the principal part of the repast, and not, like fruits generally, as dessert."
Perhaps this ruling and its explanation are why the FDA categorizes tomato juice with vegetable juices. People just seem to consider tomatoes to be vegetables, even if they are technically fruit. When you go to the grocery store, you don’t generally find tomatoes next to the apples and bananas. Instead, they are placed with other vegetables.
Maybe it’s actually all about salt…
You might be tempted to put salt on a tomato or on cucumber slices or green beans for that matter. These items are all technically fruits, but we treat them like vegetables. You would never think of putting salt on raw fruit like oranges or strawberries. I guess this is why our cooks, our stores, and even our courts consider tomatoes to be vegetables even though technically they are not.
from:http://factorfictionblog.com/2008/01/29/is-a-tomato-a-fruit.aspx