Sentence word

A sentence word (also called a one-word sentence) is a single word that forms a full sentence.
Henry Sweet described sentence words as 'an area under one's control' and gave words such as "Come!", "John!", "Alas!", "Yes." and "No." as examples of sentence words. The Dutch linguist J. M. Hoogvliet described sentence words as "volzinwoorden". They were also noted in 1891 by Georg von der Gabelentz, whose observations were extensively elaborated by Hoogvliet in 1903; he does not list "Yes." and "No." as sentence words. Wegener called sentence words "Wortsätze".
Single-word utterances and child language acquisition.
One of the predominant questions concerning children and language acquisition deals with the relation between the perception and the production of a child's word usage. It is difficult to understand what a child understands about the words that they are using and what the desired outcome or goal of the utterance should be.
Holophrases are defined as a "single-word utterance which is used by a child to express more than one meaning usually attributed to that single word by adults." The holophrastic hypothesis argues that children use single words to refer to different meanings in the same way an adult would represent those meanings by using an entire sentence or phrase. There are two opposing hypotheses as to whether holophrases are structural or functional in children. The two hypotheses are outlined below.
Structural holophrastic hypothesis.
The structural version argues that children's “single word utterances are implicit expressions of syntactic and semantic structural relations.” There are three arguments used to account for the structural version of the holophrastic hypothesis: The comprehension argument, the temporal proximity argument, and the progressive acquisition argument.
→ Child: "Daddy" (holding pair of fathers pants)
The usage of 'Daddy' and 'Bai' used in close proximity are seen to represent a child's knowledge of linguistic relations; in this case the relation is the 'possessive'. This argument is seen as having insufficient evidence as it is possible that the child is only switching from one way to conceptualize pants to another. It is also pointed out that if the child had knowledge of linguistic relationships between words, then the child would combine the words together, instead of using them separately.
Functional holophrastic hypothesis.
Functionalists doubt whether children really have structural knowledge, and argue that children rely on gestures to carry meaning (such as declarative, interrogative, exclamative or vocative). There are three arguments used to account for the functional version of the holophrastic hypothesis: The intonation argument, the gesture argument, and the predication argument.
→ Child: "Ball." (flat intonation) - Can mean "That is a ball."
→ Child: "Milk." (points at milk jug) - could mean “That is milk.” 
Single-word utterances and adult usage.
While children use sentence words as a default strategy due to lack of syntax and lexicon, adults tend to use sentence words in a more specialized way, generally in a specific context or to convey a certain meaning. Because of this distinction, single word utterances in children are called 'holophrases', while in adults, they are called 'sentence words'. In both the child and adult use of sentence words, context is very important and relative to the word chosen, and the intended meaning.
Sentence word formation.
Many sentence words have formed from the process of devaluation and semantic erosion. Various phrases in various languages have devolved into the words for "yes" and "no" (which can be found discussed in detail in yes and no), and these include expletive sentence words such as "Well!" and the French word "Ben!" (a parallel to "Bien!").
However, not all word sentences suffer from this loss of lexical meaning. A subset of sentence words, which Fonagy calls "nominal phrases", exist that retain their lexical meaning. These exist in Uralic languages, and are the remainders of an archaic syntax wherein there were no explicit markers for nouns and verbs. An example of this is the Hungarian language "Fecske!", which transliterates as "Swallow!", but which has to be idiomatically translated with multiple words "Look! A swallow!" for rendering the proper meaning of the original, which to a native Hungarian speaker is neither elliptical nor emphatic. Such nominal phrase word sentences occur in English as well, particularly in telegraphese or as the rote questions that are posed to fill in form data (e.g. "Name?", "Age?").
Sentence word syntax.
A sentence word involves invisible covert syntax and visible overt syntax. The invisible section or "covert" is the syntax that is removed in order to form a one word sentence. The visible section or "overt" is the syntax that still remains in a sentence word. Within sentence word syntax there are 4 different clause-types: Declarative (making a declaration), exclamative (making an exclamation), vocative (relating to a noun), and imperative (a command).
The words in bold above demonstrate that in the overt syntax structures, there are words that can be omitted in order to form a covert sentence word.
Distribution cross-linguistically.
Other languages use sentence words as well.