A compound is a lexeme (a word) that consists of more than one other lexeme. An endocentric compound consists of a head, i.e. the categorical part that contains the basic meaning of the whole compound, and modifiers, which restrict this meaning. For example, the English compound doghouse, where house is the head and dog is the modifier, is understood as a house intended for a dog. Endocentric compounds tend to be of the same part of speech (word class) as their head, as in the case of doghouse. (Such compounds were called karmadharaya in the Sanskrit tradition.)
Exocentric compounds do not have a head, and their meaning often cannot be transparently guessed from its constituent parts. For example, the English compound white-collar is neither a kind of collar nor a white thing. In an exocentric compound, the word class is determined lexically, disregarding the class of the constituents. For example, a must-have is not a verb but a noun. English language allows several types of combinations of different word classes:
N + N lipstick , teapot
A + N fast food , soft drink
V + N breakfast , sky-dive
N + V sunshine , babysit
N + A capital-intensive , waterproof
A + A deaf-mute , bitter-sweet
Like derivational rules, compounding rules may differ in productivity. In English, the N + N rule/pattern is extremely productive, so that novel compounds are created all he time and are hardy noticed. By contrast, the V + N rule/pattern is unproductive and limited to a few lexically listed items. Apart from endocentric and exocentric compounds there is another type of compound which requires an interpretation different from the ones introduced so far. Consider the hyphenated words in the examples below:
a. singer-songwriter
scientist-explorer
poet-translator
hero-martyr
b. the doctor-patient gap
the nature-nurture debate
a modifier-head structure
the mind-body problem
Both sets of words are characterized by the fact that none of the two members of the compound seems in any sense more important than the other. They could be said to have two semantic heads, none of them being subordinate to the other. Given that no member is semantically prominent, but both members equally contribute to the meaning of the compound, these compounds have been labeled copulative compounds (or dvandva compounds in Sanskrit grammarian terms).
Why are the copulative compounds in (a & b) divided into two different sets (a) and (b)? The idea behind this differentiation is that copulatives fall into two classes, depending on their interpretation. Each form in (a) refers to one entity that is characterized by both members of the compound. A poet-translator, for example, is a person who is both as a poet and a translator. This type of copulative compound is sometimes called appositional compound. By contrast, the dvandvas in (b) denote two entities that stand in a particular relationship with regard to the following noun. The particular type of relationship is determined by the following noun. The doctor-patient gap is thus a gap between doctor and patient, the nature-nurture debate is a debate on the relationship between nature and nurture, and so on. This second type of copulative compound is also known as coordinative compound. If the noun following the compound allows both readings, the compound is in principle ambiguous. Thus a scientist-philosopher crew could be a crew made up of scientist-philosophers, or a crew made up of scientists and philosophers. It is often stated that dvandva compounds are not very common in English (e.g. Bauer 1983:203), but in a more recent study by Olson (2001) hundreds of attested forms are listed, which shows that such compounds are far from marginal.
The above mentioned word formation processes are the most frequent or important in the English language, but it is rarely the case that only one process occurs in one word. Words can be loaned and then back formed, later on gaining an affix. There are practically no boundaries to those processes other that human ingenuity.