The term slave is gender-independent: slaves can be male or female and of any sexual orientation. In English, if a slave's gender must be indicated, use a phrase like "female slave" or "male slave", or the common compound terms slave-girl or slave-boy. Some male slaves are 'sissified' - i.e. they like to be treated in a traditionally feminine manner.
Casual Usage and Variation
Some people in a relationship enjoy the psychological impact of calling their partner slave or having their partner call them slave. Such usage is not by itself an indication that they are a slave any more than calling a partner 'bitch' indicates that they are a female dog.
The term slave is widely used, as it has a certain self-affirming weight. The difference between submissive and slave is the degree of submission. There is considerable debate over the exact definition of "slave".
The slave has usually given the right for their owner to exercise authority over them in some sense, within a relationship that may extend to a full time, 24 hours a day, 7 days a week. Connotatively it refers to a person who has surrendered their personal property and freedoms and become the property or chattel of their owner or Master. Strictly speaking, to be a slave, one must be owned but many people searching for an owner will call themselves "slave" as an indication of the position they are hoping to eventually have.
Some people draw up "slave contracts" that define the relationship, but these usually have no legal weight and are not intended to be used in court.
After signing a slave contract, many people celebrate the commitment with a "collaring ceremony", which can be simple or elaborate and friends are usually invited. The slave then wears a "collar", which symbolizes their status. The collar may be an actual piece of neckwear, or may be a bracelet or other piece of jewelry that symbolizes their slavery. Some collars are not removed until the relationship is disolved, although some slaves have a "formal" and a "subdued" collar for work and vanilla situations.