compound is a word composed of more than one free morphemes.

English compounds may be classified in several ways, such as the word classes or the semantic relationship of their components.

Most English compound nouns are noun phrases (= nominal phrases) that include a noun modified by adjectives or attributive nouns. Due to the English tendency towards conversion, the two classes are not always easily distinguished. Most English compound nouns that consist of more than two words can be constructed recursively by combining two words at a time. The compound science fiction writer, for example, can be constructed by combining science and fiction, and then combining the resulting compound with writer. Some compounds, such as salt and pepper or mother-of-pearl, can not be constructed in this way, however.
Types of compound nouns
Since English is a mostly analytic language, unlike most other Germanic languages, it creates compounds by concatenating words without case markers. As in other Germanic languages, the compounds may be arbitrarily long. However, this is obscured by the fact that the written representation of long compounds always contains blanks. Short compounds may be written in three different ways, which do not correspond to different pronunciations, however:
The solid or closed form in which two usually moderately short words appear together as one. Solid compounds most likely consist of short (monosyllabic) units that often have been established in the language for a long time. Examples are housewife, lawsuit, wallpaper, etc.
The hyphenated form in which two or more words are connected by a hyphen. Compounds that contain affixes, such as house-build(er) and single-mind(ed)(ness), as well as adjective-adjective compounds and verb-verb compounds, such as blue-green and freeze-dry, are often hyphenated. Compounds that contain particles, such as mother-of-pearl and salt-and-pepper, are also often hyphenated.
The open or spaced form consisting of newer combinations of usually longer words, such as distance learning, player piano, lawn tennis, etc.
Usage in the US and in the UK differs and often depends on the individual choice of the writer rather than on a hard-and-fast rule; therefore, open, hyphenated, and closed forms may be encountered for the same compound noun, such as the triplets container ship/container-ship/containership and particle board/particle-board/odd-looking particleboard.

In addition to this native English compounding, there is the classical type, which consists of words derived from Latin, as horticulture, and those of Greek origin, such as photography, the components of which are in bound form (connected by connecting vowels, which are most often -i- and -o- in Latin and Greek respectively) and cannot stand alone.
Analyzability (transparency)
In general, the meaning of a compound noun is a specialization of the meaning of its head. The modifier limits the meaning of the head. This is most obvious in descriptive compounds, also known as karmadharaya compounds, in which the modifier is used in an attributive or appositional manner. A blackboard is a particular kind of board which is (generally) black, for instance.

In determinative compounds, however, the relationship is not attributive. For example, a footstool is not a particular type of stool that is like a foot. Rather, it is a stool for one's foot or feet. (It can be used for sitting on, but that is not its primary purpose.) In a similar manner, the office manager is the manager of an office, an armchair is a chair with arms, and a raincoat is a coat against the rain. These relationships, which are expressed by prepositions in English, would be expressed by grammatical case in other languages. Compounds of this type are also known as tatpurusha compounds.

Both of the above types of compounds are called endocentric compounds because the semantic head is contained within the compound itself -- a blackboard is a type of board, for example, and a footstool is a type of stool.

However, in another common type of compound, the exocentric or bahuvrihi compound, the semantic head is not explicitly expressed. A redhead, for example, is not a kind of head, but is a person with a red head. Similarly, a blockhead is also not a head, but a person with a head that is as hard and unreceptive as a block (i.e. stupid). And someone who is barefoot is not a foot -- they're someone with a foot that is bare. And, outside of veterinary surgery, a lionheart is not a type of heart, but a person with a heart like a lion (in its bravery, courage, fearlessness, etc.).

Note in general the way to tell the two apart:
Can you paraphrase the meaning of the compound "[X . Y]" to A person/thing that is a Y, or ... that does Y, if Y is a verb (with X having some unspecified connection)? This is an endocentric compound.
Can you paraphrase the meaning if the compound "[X . Y]" to A person/thing that is with Y, with X having some unspecified connection? This is an exocentric compound.
Exocentric compounds occur more often in adjectives than nouns. A barefoot girl, for example, is not a girl that is a bare foot, but a girl with a bare foot. Similarly, a V-8 car is a car with a V-8 engine rather than a car that is a V-8, and a twenty-five-dollar car is a car with a worth of $25, not a car that is $25. The compounds shown here are bare, but more commonly, a suffixal morpheme is added, esp. -ed. Hence, a two-legged person is a person with two legs, and this is exocentric.

On the other hand, endocentric adjectives are also frequently formed, using the suffixal morphemes -ing or -er/or. A car-carrier is a clear endocentric determinative compound: it is a thing that is a carrier of cars. The related adjective, car-carrying, is also endocentric: it refers to an object which is a carrying-thing (or equivalent, which does carry).

These types account for most compound nouns, but there are other, rarer types as well. Coordinative, copulative or dvandva compounds combine elements with a similar meaning, and the compound meaning may be a generalization instead of a specialization. Bosnia-Herzegovina, for example, is the combined area of Bosnia and Herzegovina, but a fighter-bomber is an aircraft that is both a fighter and a bomber. Iterative or amredita compounds repeat a single element, to express repetition or as an emphasis. Day-by-day and go-go-go are examples of this type of compound, which has more than one head.

Analyzability may be further limited by cranberry morphemes and semantic changes. For instance, the word butterfly, commonly thought to be a metathesis for flutter by, which the bugs do, is actually based on an old bubbe-maise that butterflies are petite witches that steal butter from windowsills. Cranberry is a part translation from Low German, which is why we cannot recognize the element cran (from the Low German kraan or kroon, "crane"). The ladybird or ladybug was named after the Christian expression "our Lady, the Virgin Mary".

In the case of verb+noun compounds, the noun may be either the subject or the object of the verb. In playboy, for example, the noun is the subject of the verb (the boy plays), whereas it is the object in callgirl (someone calls the girl).
Sound patterns
A black board is any board that is black, and equal prosodic stress can be found on both elements (or, according to psycholinguist Steven Pinker, the second one is accented more heavily.) A blackboard, the compound, may have started out as any other black board, but now is a thing that is constructed in a particular way, of a particular material and serves a particular purpose; the word is clearly accented on the first syllable.

Sound patterns, such as stresses placed on particular syllables, may indicate whether the word group is a compound or whether it is an adjective-+-noun phrase. A compound usually has a falling intonation: "bláckboard," the "Whíte House", as opposed to the phrases "bláck bóárd". (Note that this rule does not apply in all contexts. For example, the stress pattern "whíte house" would be expected for the compound, which happens to be a proper name, but it is also found in the emphatic negation "No, not the black house; the white house!"
regards