The fact of being who or what a person or thing is (Oxford Dictionaries)

Sounds pretty simple when you say it that way.  Yet, there is a vast amount of academic research and countless studies on the topic of identity...its definition, how it forms--is nurtured or stunted and its significance to individuals, and their interactions within society.
Here are just a few resources to guide our understanding of the LGBTQAI identity process
Cass Identity Model 
D'Augelli's Identity Model 
Stages of Transsexual and Transgender Identity Formation 
A little something more entertaining about the gender identity distinction