"Existential Programming" is a style of computer software development that embraces the philosophical notion that "existence precedes essence" such that a single "type-less" data model can simultaneously support multiple "strongly typed" models. - Bruce Wallace, 2006
By applying the motto from Existentialism that “existence precedes essence”, a common approach is possible to topics as diverse as mixins, data integration using semantic mapping, code integration of uncooperative APIs and Frameworks, dynamic object "roles", open classes and external methods, ontology mediation, and object evolution & class/API versioning.
In a nutshell, Existential Programming proposes that one develop data models, object models, programming frameworks, etc. without imposing a single ontology, as would normally be the case; in other words, without imposing a single master E/R data model, O/O class hierarchy, business domain model, etc.
Techniques are proposed that would allow objects & relational entities to, not only evolve their strong data type over time, but actually embody multiple strong data types simultaneously. When objects are independent of a single "strong data type", they can integrate multiple "strongly typed" data models. This differs from weakly-typed programming, or completely type-less programming (as can be found in scripting languages like, say, JavaScript). Ironically, it takes a "type-less" foundation (like, say, Javascript) to really seriously do strong types in a "multi-cultural" world.
The Zen-like thought here is: Until one can create a type-less object, one can not create an object which can be multiple types simultaneously.