A waiver is the voluntary relinquishment or surrender

Surrender (spirituality and psychology)

To surrender in spirituality and religion means that a believer completely gives up his own will and subjects his thoughts, ideas, and deeds to the will and teachings of a higher power. It may also be contrasted with Submission...

 of some known right

Rights

Rights are legal, social, or ethical principles of freedom or entitlement; that is, rights are the fundamental normative rules about what is allowed of people or owed to people, according to some legal system, social convention, or ethical theory...

 or privilege

Privilege

A privilege is a special entitlement to immunity granted by the state or another authority to a restricted group, either by birth or on a conditional basis. It can be revoked in certain circumstances. In modern democratic states, a privilege is conditional and granted only after birth...

.

While a waiver is often in writing, sometimes a person's actions can act as a waiver. An example of a written waiver is a disclaimer

Disclaimer

A disclaimer is generally any statement intended to specify or delimit the scope of rights and obligations that may be exercised and enforced by parties in a legally recognized relationship...

, which becomes a waiver when accepted. Other names for waivers are exculpatory clauses, releases, or hold harmless clauses.

Sometimes the elements of "voluntary" and "known" are established by a legal fiction

Legal fiction

A legal fiction is a fact assumed or created by courts which is then used in order to apply a legal rule which was not necessarily designed to be used in that way...

.