Definition of the state of an object in OOP
You can take a look to (and cite) the book "Object-Oriented Analysis and Design" by G. Booch:


  ... An object is an entity that has state, behavior, and identity. The structure and
  behavior of similar objects are defined in their common class. The terms instance
  and object are interchangeable.
  
  We will consider the concepts of state, behavior, and identity in more detail in the
  sections that follow.
  ...


And there is a whole subsection that describes the concept of state:


  ... From this example, we may form the following low-level definition.
  
  The state of an object encompasses all of the (usually static) properties of the
  object plus the current (usually dynamic) values of each of these properties
  ...