The answer is multitudinous, as there are innumerable ways to define one's self (or oneself, but is there really only one self?). The context is all - I am a daughter, a niece, a scientist, a romantic, a writer, a teacher, a thinker, an activist, etc., etc. to ad nauseum. To my professional colleagues, I 'define' myself one way, to my students another, to my family and friends many other ways.
Perhaps, focusing in a bit tighter, the question can be how do I define myself to myself? I can answer the same to that question as well, particularly as I do not want to confine myself to being either-or.
A person is too unique, too disparate to be 'defined' as a simple collection of words or acts or behaviours. Yet, to ease comprehension and classification, to fulfill the duty of language and communication, a pigeonhole is required. To that need, I respond thusly:
I am me.