The term "vegetable" generally means the edible parts of plants. The definition of the word is traditional rather than scientific, however. Therefore the usage is somewhat arbitrary and subjective, as it is determined by individual cultural customs of food selection and food preparation.
Generally speaking, a herbaceous plant or plant part which is regularly eaten as unsweetened or salted food by humans is considered to be a vegetable. Mushrooms belong to the biological kingdom Fungi, not the plant kingdom, and yet they are also generally considered to be vegetables, at least in the retail industry. Nuts, seeds, grains, herbs, spices and culinary fruits are usually not considered to be vegetables, even though all of them are edible parts of plants.
In general, vegetables are those plant parts that are regarded as being suitable to be part of savory or salted dishes, rather than sweet dishes. However there are many exceptions, such as for example the pumpkin, which can be eaten as a savory dish, a vegetable, but which can also be sweetened and served in a pie as a dessert.
For many different kinds of vegetables, please see the list of vegetables.
The word "vegetable", in its modern usage, is strictly a culinary term, and is not in any way a botanical or scientific term. The word "fruit" on the other hand can be a culinary term, or a botanical term, and these two usages are quite different.
Botanically speaking, fruits are fleshy reproductive organs of plants, the ripened ovaries containing one or many seeds. Thus, many botanical fruits are not edible at all, and some are actually extremely poisonous.
In a culinary sense however, the word "fruit" is only applied to those botanical fruits or similar plant parts which are both edible and palatable, and which in addition are considered suitable to be a sweet or dessert food, such as strawberries, peaches, plums, etc.
In contrast to this, a number of edible botanical fruits, including the tomato, the eggplant, and the bell pepper are not considered to be a sweet or dessert food, are not routinely used with sugar, but instead are almost always used as part of a savory dish, and are salted. This is the reason that they are labeled as "vegetables". Thus in a scientific context, a plant part may correctly be termed a "fruit", even though it is used in cooking or food preparation as a vegetable.
The question "The tomato: is it a fruit, or is it a vegetable?" found its way into the United States Supreme Court in 1893. The court ruled unanimously in Nix v. Hedden that a tomato is correctly identified as, and thus taxed as, a vegetable, for the purposes of the 1883 Tariff Act on imported produce. The court did acknowledge however that botanically speaking, a tomato is a fruit not a vegetable
"Vegetable" comes from the Latin vegetabilis (animated) and from vegetare (enliven), which is derived from vegetus (active), in reference to the process of a plant growing. This in turn derives from the Proto-Indo-European base *weg- or *wog-, which is also the source of the English wake, meaning "become (or stay) alert".
The word is still sometimes used as an archaic literary term for any plant, as in vegetable matter, vegetable kingdom.
The green color of leafy vegetables is due to the presence of the green pigment chlorophyll. Chlorophyll is affected by pH and changes to olive green in acid conditions, and bright green in alkaline conditions. Some of the acids are released in steam during cooking, particularly if cooked without a cover.
The red/blue coloring of some fruits and vegetables (e.g. blackberries and red cabbage) are due to anthocyanins, which are sensitive to changes in pH. When pH is neutral, the pigments are purple, when acidic, red, and when alkaline, blue. These pigments are very water soluble.