The most naive OO definition might be:
A language feature that bundles data and methods together.You might extend that to say that it hides the data from public consumption, but that part muddies up the water, as the Wikipedia article demonstrates. My favorite example of Encapsulation is a Queue class. You get push, pop, and peek operations to call, but you don't know what data structure the uses to implement those operations. It could be an array, it could be a linked list, whatever. In this we can easily see the beauty and the power of encapsulation: "data" and "methods" together.
But wait, what did I actually encapsulate in that queue? Was it the "data"? I pass my data into and out of the Queue, and the Queue hides its implementing data structure from me. Maybe it's a Queue<string>, and I'm all: q.Push("encapsulation"); q.Push("is"); q.Push("about"); q.Push("data"); Assert.AreEqual("encapsulation", q.Pop()); For me the data are those strings, but those strings are clearly not what the Queue is encapsulating!
That word "data" in our definition is a tricky one. It can be applied to too many things to be really useful. But does replacing "data" with "data structure" in the definition fix the problem?
A language feature that bundles data structures and methods together.It clears it up, but it introduces another problem. For example, what if my object is a database gateway? Certainly there's a data structure somewhere in that database, but my object isn't directly encapsulating that! No, it's probably "encapsulating" ADO.NET procedural calls, or some other data access library. The procedural calls are neither data nor data structure... So could it be that thinking about data is completely misleading?
A language feature that bundles implementation details and methods together.This is a rather large step though! Instead of just talking about data, or data structures, this now includes just about anything in the definition of things that can be bundled with methods to achieve encapsulation! Maybe there is some value in restricting what the word "encapsulation" applies to, but if there is, I doubt it's something that is going to prove useful for Software Engineers. So while I admit this definition could be a perversion of the word "encapsulation," I find it more useful.
The other definition Wikipedia gives for encapsulation, which I've neglected to tell you about until now, is "A language mechanism for restricting access to some of the object's components." This is more similar to the definition I just ended on. I take some issue with the word "restricting" and the word "components" is ambiguous enough to be a problem. But I don't think it's a stretch to think "components" could include both data and dependencies.
So, perhaps we've arrived at a better understanding of encapsulation. One that recognizes that data is not the all important concept. The next step I'd like to take is to extend this slightly deeper understanding outside the realm of data structures and into more typical "business" scenarios. That will be the next post.