Editing Is-a (section)

{{lowercase title}}
{{Short description|Subsumption relationship between abstractions}}
{{multiple issues|
{{essay-like|date=August 2018}}
{{technical|date=August 2018}}
}}

In [[knowledge representation and reasoning|knowledge representation]], [[ontology components]] and [[ontology engineering]], including for [[object-oriented programming]] and [[Object-oriented design|design]], '''is-a''' (also written as '''is_a''' or '''is a''') is a [[wikt:subsume|subsumptive]]{{efn|See [[Liskov substitution principle]].}} relationship between [[abstractions]] (e.g., [[type (disambiguation)|types]], [[class (knowledge representation)|classes]]), wherein one [[Class (knowledge representation)|class]] ''A'' is a [[subclass (knowledge representation)|subclass]] of another class ''B'' (and so ''B'' is a [[superclass (knowledge representation)|superclass]] of ''A'').
In other words, type A is a [[subtyping|subtype]] of type B when A's [[Formal specification|specification]] implies B's specification. That is, any object (or class) that satisfies A's specification also satisfies B's specification, because B's specification is weaker.<ref>{{cite web|title=Subtypes and Subclasses|url=http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-170-laboratory-in-software-engineering-fall-2005/lecture-notes/lec14.pdf|publisher=MIT OCW|access-date=2 October 2012}}</ref>

For example, a cat 'is a[n]' animal, but not vice versa. All cats are animals, but not all animals are cats.
Behaviour that is relevant to all animals is defined on an animal class, whereas behaviour that is relevant only for cats is defined in a cat class. By defining the cat class as 'extending' the animal class, all cats 'inherit' the behaviour defined for animals, without the need to explicitly code that behaviour for cats.