Hyponymy and hypernymy

In linguistics, hyponymy (from Greek ὑπό, "hupó", "under", and ὄνυμα, "ónuma", "name") is a semantic relation between a hyponym denoting a subtype and a hypernym or hyperonym denoting a supertype. In other words, the semantic field of the hyponym is included within that of the hypernym. In simpler terms, a hyponym is in a "type-of" relationship with its hypernym. For example: "pigeon", "crow", "eagle", and "seagull" are all hyponyms of "bird", their hypernym; which itself is a hyponym of "animal", its hypernym.
Hypernymy or hyperonymy (from Greek ὑπέρ, "hupér", "over", and ὄνυμα, "ónuma", "name") is the converse of hyponymy.
Other names for hypernym include umbrella term and blanket term. A synonym of co-hyponym based on same tier (and not hyponymic) relation is allonym (which means "different name").
A hyponym refers to a "type". A meronym refers to a "part". For example, a hyponym of "tree" is "pine tree" or "oak tree" (a type of tree), but a meronym of "tree" is "bark" or "leaf" (a part of tree).
Hyponyms and hypernyms.
Hyponymy shows the relationship between a generic term (hypernym) and a specific instance of it (hyponym). A hyponym is a word or phrase whose semantic field is more specific than its hypernym. The semantic field of a hypernym, also known as a superordinate, is broader than that of a hyponym. An approach to the relationship between hyponyms and hypernyms is to view a hypernym as consisting of hyponyms. This, however, becomes more difficult with abstract words such as "imagine", "understand" and "knowledge". While hyponyms are typically used to refer to nouns, it can also be used on other parts of speech. Like nouns, hypernyms in verbs are words that refer to a broad category of actions. For example, verbs such as "stare", "gaze", "view" and "peer" can also be considered hyponyms of the verb "look", which is their hypernym.
Hypernyms and hyponyms are asymmetric. Hyponymy can be tested by substituting X and Y in the sentence "X is a kind of Y" and determining if it makes sense. For example, "A screwdriver is a kind of tool" makes sense, but not "A tool is a kind of screwdriver".
Strictly speaking, the meaning relation between hyponyms and hypernyms applies to lexical items of the same word class (or parts of speech), and holds between senses rather than words. For instance, the word "screwdriver" used in the previous example refers to the tool for turning a screw, and not to the drink made with vodka and orange juice.
Hyponymy is a transitive relation, if X is a hyponym of Y, and Y is a hyponym of Z, then X is a hyponym of Z. For example, "violet" is a hyponym of "purple" and "purple" is a hyponym of "color"; therefore "violet" is a hyponym of "color". A word can be both a hypernym and a hyponym: for example "purple" is a hyponym of color but itself is a hypernym of the broad spectrum of shades of purple between the range of "crimson" and "violet".
The hierarchical structure of semantic fields can be mostly seen in hyponymy. They could be observed from top to bottom, where the higher level is more general and the lower level is more specific. For example, "living things" will be the highest level followed by "plants" and "animals", and the lowest level may comprise "dog", "cat" and "wolf".
Under the relations of hyponymy and incompatibility, taxonomic hierarchical structures too can be formed. It consists of two relations; the first one being exemplified in "An X is a Y" (simple hyponymy) while the second relation is "An X is a kind/type of Y". The second relation is said to be more discriminating and can be classified more specifically under the concept of taxonomy.
Co-hyponyms.
If the hypernym Z consists of hyponyms X and Y, X and Y are identified as co-hyponyms. Co-hyponyms are labelled as such when separate hyponyms share the same hypernym but are not hyponyms of one another, unless they happen to be synonymous. For example, "screwdriver", "scissors", "knife", and "hammer" are all co-hyponyms of one another and hyponyms of "tool", but not hyponyms of one another: *"A hammer is a type of knife" is false.
Co-hyponyms are often but not always related to one another by the relation of incompatibility. For example, "apple", "peach" and "plum" are co-hyponyms of "fruit". However, an "apple" is not a "peach", which is also not a "plum". Thus, they are incompatible. Nevertheless, co-hyponyms are not necessarily incompatible in all senses. A "queen" and "mother" are both hyponyms of "woman" but there is nothing preventing the "queen" from being a "mother". This shows that compatibility may be relevant.
Autohyponyms.
A word is an autohyponym if it is used for both a hypernym and its hyponym. For example, the word "dog" describes both the species "Canis familiaris" and male individuals of "Canis familiaris", so it is possible to say "That dog isn't a dog, it's a bitch" ("That hypernym Z isn't a hyponym Z, it's a hyponym Y"). The term "autohyponym" was coined by linguist Laurence R. Horn in a 1984 paper, "Ambiguity, negation, and the London School of Parsimony." Linguist Ruth Kempson had already observed that if there are hyponyms for one part of a set but not another, the hypernym can complement the existing hyponym by being used for the remaining part. For example, fingers describe all digits on a hand, but the existence of the word thumb for the first finger means that fingers can also be used for "non-thumb digits on a hand". Autohyponymy is also called "vertical polysemy".
Horn called this "licensed polysemy", but found that autohyponyms also formed even when there is no other hyponym. Yankee is autohyponymous because it is a hyponym (native of New England) and its hypernym (native of the United States), even though there is no other hyponym of Yankee (as native of the United States) that means "not a native of New England". Similarly, the verb to drink (a beverage) is a hypernym for to drink (an alcoholic beverage).
In some cases, autohyponyms duplicate existing, distinct hyponyms. The hypernym "smell" (to emit any smell) has a hyponym "stink" (to emit a bad smell), but is autohyponymous because "smell" can also mean "to emit a bad smell", even though there is no "to emit a smell that isn't bad" hyponym.
Hyperonym or hypernym.
Both "hyperonym" and "hypernym" are in use in linguistics. The form "hypernym" takes the "-o-" of "hyponym" as a part of "hypo" in the same way as in the contrast between "hypertension" and "hypotension". However, etymologically the "-o-" is part of the Greek stem "ónoma". In other combinations with this stem, e.g. "synonym", it is never elided. Therefore, "hyperonym" is etymologically more faithful than "hypernym". "Hyperonymy" is used, for instance, by John Lyons, who does not mention "hypernymy" and prefers "superordination". The nominalization "hyperonymy" is rarely used, because the neutral term to refer to the relationship is "hyponymy". A practical reason to prefer "hyperonym" is that "hypernym" is in its spoken form hard to distinguish from "hyponym" in most dialects of English.
Usage.
Computer science often terms this relationship an "is-a" relationship. For example, the phrase "Red is-a color" can be used to describe the hyponymic relationship between "red" and "color".
Hyponymy is the most frequently encoded relation among synsets used in lexical databases such as WordNet. These semantic relations can also be used to compare semantic similarity by judging the distance between two synsets and to analyse anaphora.
As a hypernym can be understood as a more general word than its hyponym, the relation is used in semantic compression by generalization to reduce a level of specialization.
The notion of hyponymy is particularly relevant to language translation, as hyponyms are very common across languages. For example, in Japanese the word for older brother is , and the word for younger brother is . An English-to-Japanese translator presented with a phrase containing the English word "brother" would have to choose which Japanese word equivalent to use. This would be difficult, because abstract information (such as the speakers' relative ages) is often not available during machine translation.