Abstraction

"An abstraction denotes the essential characteristics of an
object that distinguish it from all other kinds of object and thus
provide crisply defined conceptual boundaries, relative to the
perspective of the viewer."

-- [Booch, 1991]



"[A] simplified description, or specification, of a system that
emphasizes some of the system's details or properties while
suppressing others. A good abstraction is one that emphasizes
details that are significant to the reader or user and suppress
details that are, at least for the moment, immaterial or
diversionary."

-- [Shaw, 1984]


Abstraction, as a process, denotes the extracting of the essential
details aboutan item, or a group of items,while ignoring the
inessential details.

Abstraction, as an entity,
denotes a model, a view, or some other focused representation for
an actual item. Abstraction is most often used as a complexity
mastering technique.


Abstraction refers to the act of representing essential features
without including the background details or explanations.
Classes use the concept of abstraction and are defined as a
list of abstract attributes.

Example:

An electrician and an interior decorator will look at the same building each in two different ways. The decorator will probably view the building primarily as a series of rooms. However, the electrician will view it as a series of walls and floors. Walls are in both their views, but serve a different purpose, have a different priority, and relate in different ways.

Thus, the "abstraction" used by the decorator may be rooms which are composed/comprised of walls and floors/ceilings. But the electrician's view may have the walls be the primary abstraction, with room information incidental, perhaps only to locate the proper wall when walking around.

Further, in the electrician's view, walls are a complex 3D structure with their own little world inside, while the interior decorator may view them mostly as 2D flat panels. Both have walls in their mental abstractions, but view and relate them differently.