The vocabulary of gender identity is evolving as societies move away from limited labels of male and female towards more fluid self-expression.
Even Facebook has changed: since 2014, it has allowed its users to choose from dozens of identities such as agender, bigender, gender questioning and non-binary.
Here is a look at some of the terminology.
- Sex and gender -
These words are often used interchangeably but they are very different: sex refers to the physical characteristics one is born with -- usually male or female -- while gender refers to socially-constructed ideas associated with the sexes.
"Sex is assigned at birth and written on a birth certificate, usually based on the appearance of ... external anatomy and on a binary vision of sex," says transgender rights group ILGA Europe.
Gender refers to "a social construct which places cultural and social expectations on individuals based on their assigned sex," it says.
Gender norms vary from society to society and can change over time.
A person’s gender identity -- their profound psychological sense of who they are -- often matches the sex they are born into, but not always.
- Cisgender and transgender -
Cisgender is the term used when there is such a match, for example a baby girl who grows up to identify as a woman.
"Cis-" is a Latin-derived prefix that means "on this side of" while "trans-" means "on the other side of."
In this way transgender, or just trans, refers to someone whose gender identity is different to their sex at birth.
There are many expressions of transgender, such as transman, transwoman and third gender.
Some transgender people try surgery or hormone treatment to align their body with their gender identity, from where derives the word transition.
In many countries, transgender people are harassed, criminalized or sent to psychiatric institutions, the United Nations' Free and Equal campaign says.
There are no reliable global statistics on the number of people who identify as transgender. In a 2014 report, Amnesty International cites surveys that suggest there could be as many as 1.5 million transgender people in the European Union.
A UN Development Program report of 2014 says a study of the Asia Pacific region suggested that there are more than nine million trans people in the region.
- Intersex -
Intersex is a broad term encompassing people who have sex traits, such as genitals or chromosomes, that do not entirely fit with typical binary notion of either male or female.
So intersex people would have features that are neither wholly female nor wholly male or a combination or neither.
"According to experts, between 0.05 percent and 1.7 percent of the population is born with intersex traits -- the upper estimate is similar to the number of red-haired people," the UN campaign says.
People who identify as intersex are recognised on official documents in some countries including Australia, Germany, India, New Zealand and Nepal.
- LGBT, LGBTI and beyond -
While transgender and intersex have no bearing on a person's sexual orientation, they are included in the all-encompassing acronym for lesbian, gay, bisexual and trans (LGBT).
It emerged around the 1990s and is often preferred to the term "gay community".
There are many variations, for example adding an I (LGBTI) for intersex or a Q for questioning (LGBTQ) or for queer, or even both.
Falling under this broad umbrella term are a range of gender identities such as: nonbinary and agender, in which people don't identify as male or female; and bigender, someone who moves between the female and male identities.
There are also identities of sexual orientation beyond gay and lesbian such as: asexual, a person who has no physical attraction to other people; and pansexual, someone whose attractions are not limited to sex or gender identity.