Self: What one is, especially in terms of one's capacities and character; what produces one's experiences.
The Buddhists suppose the self to be a delusion, and it is true that self-knowledge, let alone complete self-knowledge, is difficult, may be painful, and may be impossible if one seeks completeness of it.
Also, it is difficult to select in one's experiences a self, but then what one considers one's self depends much on memories, fantasies and choices one (believes one) has made, and it seems reasonable to suppose that if the self exists at all, it is too complicated and large to be experienced completely at once, in one's specious present.
In any case, whether or not the self is a delusion, and whether or not one has much self-knowledge, every person in a human society is treated as playing some role with which one is to some extent identified by others, and identifies oneself with to some extent, at least in the sense of accepting judgements about oneself that others also accept ("I am the child of so-and-so, born then and there, having learned such and such").
Moreover, every person in a human society knows how to play quite a few kinds of roles - child, pupil, adult, law-abiding citizen, for example, for everyone who has grown up - and also every person has a name, that serves as a means of verbal identification in social context, and in one's thinking about one's self.
And there is in every natural language a personal pronoun - "I", which for the user of "I" draws the user's attention to the user's theory of who the user is: What one believes oneself to be (like) and how this effects one's present experiences - which are merely the consciously given surface of the self, person, mind or brain that produces it.
Then it seems that everyone who is conscious (and not totally incapacitated) has more or less constantly a body-image, that informs one about one's limbs, and their position and state, that may serve as a kind of constant aspect of one's experiencing, and as a basis for a feeling of self.
Also, whether or not the self is a delusion: one's body is not, and is given to one in experience, with many strong needs and feelings, that need very frequent attention and care.
Apart from whether or not a person really has or is a self, every sane adult person in a human society is treated as if one is a person: as if he or she has a self, that consists at least in part in having a history, beliefs, desires, experiences, roles, responsibilities, duties, freedoms, rights, personal ends and interests, and the ability to reason about himself or herself at least as if all this is so, and as if there resides inside or connected to one's body a unique entity that has all these properties; that has a free will, and has minimally adequate ideas about society and reality; that has both personal ends and impersonal ends and values, quite a few of which may be impossible, impracticable or improbable to be realized, and that may go far beyond what one knows or assumes to be facts; and that one can be held responsible for his or her actions, ideas, values, and ends, and be judged by others, and be punished or rewarded for what one is or tries to be.
And indeed, on a realistic hypothesis, this self is a coordinated set of capacities that one's brain produces to account for its experiencing, and accordingly the self is a theory that one's brain produces about what one is, may be, and would like to be, which includes a theory about the world one is in that one experiences through one's sensations.
Furthermore, it is important to see that, whether or not there is a real self, that being supposed to be a person and playing roles in a society involves being supposed to have ideas about what one is, and desires and ends about what one wants to be, and that these ideas may well be partially false, and these desires and ends may well be difficult or impossible to realize, and that in any case one's ends and ideals about oneself and the world normally go far beyond what one knows or indeed may know that one is, and also far beyond the present time, or indeed the time one may be expected to live.
The - presumed - facts just listed about a person playing roles in a society and imagining itself to be someone with a past, a present, a future, and ends and values about the world it finds itself in, show that what a person really is at any one moment is, at least to a considerable extent, a theoretical and partially hypothetical construction, that is built from beliefs that may well be false and desires that well may be impracticable and based on false ideas, and that involves many references and ideas about both the past and the future.