What does it mean to define something and what does this have to do with self-definition?
We check in the dictionary to define something and see other words. Words that if their definitions are looked up, equate to more words which use others in their definitions. The basis of words contain a working but cyclic mind state where all "definitions" hold true so long as the language they are made of binds them to relative verity. They are only words and do not define the objects or concepts themselves.
A child points at an object and asks their parent, "What is this?" The parent answers back: "This is a clock." (Or insert your favorite object here). So the item is given a name. We mistakenly take that name as a definition of the item. We point, and that item is now item name.
Later that child grows older and to that item name adds the workings and functions and various characteristics of that object. It's definition is modified, enlarged, and so the child thinks, made more real by adding knowledge.
The item is not ever really defined. It is only given a name and given descriptions of its properties or workings. These added things do not get at the essence of the object. So it is with our self.
The child pointing at the item before it knows what to call it, knows it more closely to its real definition than after it labels and categorizes the item (mistaken definition). We are given knowledge of the world and ourselves through our senses and then we try to make sense of this information. This is already one step removed from that information/item/object/being itself.
Of course this making sense of the world is required of daily living and making our way in the world...but the child has knowledge we "forgot" as adults. How does this apply to our self, who we really are, self-definition?
Information we receive about the world through our senses is not the true definition of our self. It is like the words we give to things, separate, a step removed from the thing itself. (Ideas can be used interchangeably with things here).
As we begin to see that the things we attribute to ourselves or belonging to our self such as our stories and habits, likes and dislikes, and other things that are "ours," are not in themselves a working definition of "I," our being, it dawns on us that we do not really know who or what we are. The workings and functionings (psychology) is not the definition of our self.
Definition is knowing. Directly, not through words or concepts but through being/identity. And to get there we must know the knower. That consciousness (or awareness) that is the source of our being beyond the label of "I" and its workings, THAT is definition. So the masters say: You are THAT.
So the question becomes: How do I define myself? Who's defining? And what do I take myself to be? These are good questions to ask and may make the mind stop or contemplate without an easy answer. It's not an easy question but there are those who have found a way to answer it satisfactorily (to themselves at least). So with proper faith in our ability to accomplish this we meditate and contemplate, think and don't think on this (and if we're lucky, "in between think" as well) hoping for an answer to come all the while working through various practices or suggestions of our fellow seekers.
We are given various seemingly worthwhile advice from those who have traveled farther down the path than us.
Look first at what you are not...because there's supposed to be a lot of that.
Look until you see you're not that and keep going. This is a subtractive path with no end until it ends for real in Definition.
Look at the body. Is that what you are?
Look at your response to pleasant and painful, likes and dislikes. If those were not there what would you be?
Look at thoughts and moods. How do they affect you?
Look at awareness/consciousness can you see what that is clearly?
Definition again may change as we age. As prides and various egos fall away from traumas or simply time taking its toll upon us, we lose a previously cherished self-belief, a false definition and once again redefine ourselves. "Well I'm really not this at all (stop here) but I must be that now." If we are able to stop at the "stop here" point rather than immediately go on and assume a further or replacement self-definition, there is a possibility of questioning or looking in earnest at the identification mechanism itself (which is still at least step removed from identity itself being in the mental realm). Stop at that point and ask if I am not that, then what does that make me? Who am I? You must be clear on this. If not, then your definition, though it may be a relatively correct and 'working definition,' may be actually flawed and not ultimately true. A test: If you remove it, that definition, where does that leave you?