Gender refers to an individual's personal and social identity as a man, woman or non-binary person (a person who is not exclusively a man or a woman).
Gender includes the following concepts:
gender identity, which refers to the gender that a person feels internally and individually;
gender expression, which refers to the way a person presents their gender, regardless of their gender identity, through body language, aesthetic choices or accessories (e.g., clothes, hairstyle and makeup), which may have traditionally been associated with a specific gender.
A person's gender may differ from their sex at birth, and from what is indicated on their current identification or legal documents such as their birth certificate, passport or driver's licence. A person's gender may change over time.
Some people may not identify with a specific gender.

Definition 2:
Given that the non-binary population is small, data aggregation to a two-category gender variable is sometimes necessary to protect the confidentiality of responses provided. In these cases, individuals in the category “non-binary persons” are distributed into the other two gender categories and are denoted by the “+” symbol.