What does object-oriented mean?

Despite following programming trends for nearly three decades, I have never seen a definition for the concept “object-oriented.” Instead, it is normally described through its attributes (encapsulation, polymorphism-enabling, etc); or through its applications (analysis, design, programming); or through the benefits it brings (reusability, maintainability, etc.)  Sometimes it is even described through what it is not. Quite an elusive concept!

Since I use the term repeatedly in relation to UnixObjects, I feel it is my responsibility to define it here.

object-orientedness  =  organization by containment

Other organizational principles include: organization by type, organization by time, etc.