"In the beginning, IS-A was quite simple. Today, however, there are almost as many meanings for this inheritance link as there are knowledge-representation systems."
Semantics of IS-A relationship (inheritance):
When two generics are related by an IS-A connection, the intent is usually that one is somehow related to, but less general than, the other.
(IS-A of labmda-abstraction) A triangle IS-A a ploygon --> to be read as: "To be a triangle is to be a polygon".
An IS-A connection between an individual and a generic generally means that an individual is describable by some general description. This relation is often called "instantiation."
(a predicate to an individual) e.g. if the generic is Camel and the individual is Clyde, IS-A expresses the fact that Camel(Clyde).
e.g. this is the relationship between "King" and "the King of Saudi Arabia". Generic is used to construct the individual description.
"What IS-A Is and Isn't: An Analysis of Taxonomic Links in Semantic Networks", Ronald J. Brachman 1983