In linguistics, a hyponym (from Greek hupó, "under" and onomas, "name") is a word or phrase whose semantic field is included within that of another word, its hyperonym or hypernym (from Greek hupér, "over" and onomas, "name") . In simpler terms, a hyponym shares a type-of relationship with its hypernym. For example, pigeon, crow, eagle and seagull are all hyponyms of bird (their hypernym); which, in turn, is a hyponym of animal.
Hyponymy shows the relationship between the more general terms (hypernyms) and the more specific instances of it (hyponyms). 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, hyponyms 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.
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. In addition, it should be noted that a word can be both a hypernym and a hyponym: for example purple is a hyponym of colour 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 of 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.
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, hammer are all co-hyponyms of tool, but not hyponyms of one another: ‘That hammer is a knife’ does not make sense.
Co-hyponyms are often but not always be 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.
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.
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 Ani (兄 Ani?), and the word for younger brother is Otōto (弟 Otōto?). 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.