In a story, we detect an isotopy when there is a repetition of a basic meaning trait (seme); such repetition, establishing some level of familiarity within the story, allows for a uniform reading/interpretation of it. An example of a sentence containing an isotopy is I drink some water. The two words drink and water share a seme (a reference to liquids), and this gives homogeneity to the sentence.
This concept, introduced by Greimas in 1966, had a major impact on the field of semiotics, and was redefined multiple times.Catherine Kerbrat-Orecchioni extended the concept to denote the repetition of not only semes, but also other semiotic units (like phonemes for isotopies as rhymes, rhythm for prosody, etc.).Umberto Eco showed the flaws of using the concept of "repetition", and replaced it with the concept of "direction", redefining isotopy as "the direction taken by an interpretation of the text".
The concept was highly influential and has been revisited and redefined by multiple authors, starting from Greimas, to his epigons of the Paris school, Umberto Eco, the Groupe µ, and others.
Greimas' initial definition was based on the concept of repetition (also termed recurrence or redundancy), was focused on semantics as it only regarded the repetition semes, and it stressed the role of isotopy of making possible a uniform reading of a story and resolving ambiguities. To quote his first 1966 formulation: "a redundant set of semantic categories which make possible the uniform reading of the story."
In 1980 Umberto Eco showed a flaw of using the concept of "repetition". He noted that there are cases in which an isotopy is not a repetition of a seme, like in the French sentence l'ami des simples = l'herboriste, in which ami (meaning lover, friend or fan) and simples (medicinal plants) does not appear to share a seme; to also embrace cases like this, Eco replaced the concept of "repetition" with the concept of "direction", defining isotopy more generally as "a constancy in going in a direction that a text exhibits when submitted to rules of interpretative coherence."