Gender comprises a range of differences between men and women, extending from the biological to the social. At the biological level, men and women are typically distinguished by the presence of a Y-chromosome in male cells, and its absence in female cells. At the social level, however, there is debate regarding the extent to which the various biological differences necessitate differences in social gender roles and gender identity, which has been defined as "an individual's self-conception as being male or female, as distinguished from actual biological sex."

The word "gender" has several definitions. Colloquially, it is used interchangeably with "sex" to denote the condition of being male or female, but in the social sciences it refers to specifically social differences, such as but not limited to gender identity. More recently, it has been equated with "sexual orientation" and "identity" (especially LGBT sexuality).[citation needed] People whose gender identity feels incongruent with their biological sex may refer to themselves as "intergender".

Many languages have a system of grammatical gender, a type of noun class system—nouns may be classified as masculine or feminine (for example Spanish, Hebrew, Arabic and French) and may also have a neuter grammatical gender (for example Sanskrit, German, Polish, and the Scandinavian languages). In such languages, this is essentially a convention, which may have little or no connection to the meaning of the words. Likewise, a wide variety of phenomena have characteristics termed gender, by analogy with male and female bodies (such as the gender of connectors and fasteners) or due to societal norms.