Derived from a term for slavery, Bondage has come to mean sexual slavery, particularly dealing with the simple practice of restraining one's parter to beds, posts, from ceilings, etc. Usually entails either rope (asian forms) or leather, chains, and other equipment specially designed for the purpose (western.)

Bondage considered a fetish, as enthusiasts generally prefer the thought or presence of a fully-clothed restrained partner to a fully nude and unrestrained partner.

Bondage is not to be confused with the more specific practices of B&D, the practice of restraining and then dominating, or humiliating a partner, or S&M, the practice of hurting one's partner in order to achieve sexual climax. Bondage enthusiasts generally prefer the more docile forms.

Tying up or restraining your consenting sexual partner.
Usually the restraining is done by the dom (dominant) and the sub (submissive) is the one restrained.
Sexually arousing for some, as the exchange of power is exciting, and the helplessness of being bound allows some people to really let go and enjoy themselves. Also, having complete control over a willing partner is very arousing. There are many reasons people enjoy this.

HAS NOTHING TO DO WITH VIOLENCE.
Bondage is often used in conjuction with punishment/pain play, but it is NOT the same thing!!!

The practice of being restrained for sexual gratification.

The consensual and romantic restraint of a beloved and cherished partner for mutual sexual enjoyment

The act of tying someone up for sexual pleasures

Bondage does not mean using leather objects for sexual pleasure. It means being restrianed /tied and can be done with a variety of materials - ie ropes, scarves, rubber, leather thongs, cling film, chains etc

Using restraining objects (e.g. Handcuffs) on a consenting partner during sex.

Some people confuse bondage with BSDM, but bondage and BSDM are quite different: BSDM is the act of hurting someone during sex, bondage is simply restraining them.