Most people when they refer to the object class "aircraft" refer to the head/primary definition, not to the head/primary object. And the classes people refer to are not only object classes, but also definitions. One could pose the question, is a definition part of an object, or not? To make the answer more complicated, it depends; a definition can be part of an object, but that is not always the case. It depends if the object is a physical object or a non-physical object. It also depends if it is a head/base object or one of the higher, more complex objects.

One could say that a head/primary definition is a non-physical object. It is an object we all refer to when we are speaking about aircraft, but there is no such physical object. There is no physical head/primary object called "aircraft" that all aircraft have in common. Rather what they have in common is there head/primary definition. This is not always so; for example., consider a specific species of plant, for which a physical head/primary object exists. That head/primary object might be a physical compound object, and the definition also the same, but when they became separate species that definition became more elaborate.

To keep things clear I will use the following terms :
physical head/primary object, head/primary definition, object definition.

This points to a new question. Are there only physical objects or are there also non-physical objects. If one has read carefully above, one could come to the conclusion that the answer to that question is yes. In common spoken language there are indeed non-physical objects. We refer to them as objects, while what we actually refer to are there definitions. The real objects we think we refer to do not exist in many cases. There are those that exist, but there are far more of those head/primary objects that do not exist, than there are those that do exist. To make things not more complicated I will only use the above 3 terms.

This might be rather complex for some, so to make it a bit clearer I will introduce a new term: Knowledge levels.

There are several knowledge levels. How much we understand of a certain compound object, depends on how many knowledge levels we understand. Let me give an example. Everyone can fairly easily use a mobile phone. Most people understand 2 knowledge levels at most. Most of us do not need to know the intricate s of the chip design and how the electrons work, to use the mobile phone.

Now let's try to make the above explanations a bit clearer. We will use the object chair as an example, as well as others to point out the differences.

Some time ago I was at a shop called "Pot" in the Netherlands. I saw an object from which I had no clue what it was. I knew it was a physical object, but it could be a work of art or something else. I looked for a word saying what it was, and found a ticket saying "chair." Instantly without thinking my mind looked up the definition of the word "chair," compared it to what I saw, and recognized that object as a chair. Now when I see that object or an image of it, I instantly recognize it as a chair. When we say the chair is red, then we refer to a property of an actual physical object, namely that red chair. The word "chair" refers to the head/primary definition, which all chairs have in common. The class "chair" is a class to which all chairs belong. The word "chair" refers to the head/primary definition of what a chair is, what it is meant to be used for. Each chair can have a different sub definition.