Consent occurs when one person voluntarily agrees to the proposal or desires of another. It is a term of common speech, with specific definitions as used in such fields as the law, medicine, research, and sexual relationships. Consent as understood in specific contexts may differ from its everyday meaning. For example, a person with a mental disorder, a low mental age, or under the legal age of sexual consent may willingly engage in a sexual act that still fails to meet the legal threshold for consent as defined by applicable law.

United Nations agencies and initiatives in sex education programs believe that teaching the topic of consent as part of a comprehensive sexuality education is beneficial. Types of consent include implied consent, express consent, informed consent and unanimous consent.

Types.

An expression of consent is one that is unmistakably stated, rather than implied. It may be given in writing, by speech (orally), or non-verbally, for example by a clear gesture such as a nod. Non-written express consent not evidenced by witnesses or an audio or video recording may be disputed if a party denies that it was given.

Implied consent is consent inferred from a person's actions and the facts and circumstances of a particular situation (or in some cases, by a person's silence or inaction). Examples include unambiguously soliciting or initiating sexual activity or the implied consent to physical contact by participants in a hockey game or being assaulted in a boxing match.

Informed consent in medicine is consent given by a person who has a clear appreciation and understanding of the facts, implications, and future consequences of an action. The term is also used in other contexts, such as in social scientific research, when participants are asked to affirm that they understand the research procedure and consent to it, or in sex, where informed consent means each person engaging in sexual activity is aware of any positive statuses (for sexually transmitted infections and or diseases) they might expose themselves to.

Unanimous consent, or general consent, by a group of several parties (for example, an association) is consent given by all parties.

Substituted consent, or the substituted judgment doctrine, allows a decision maker to attempt to establish the decision an incompetent person would have made if they were competent.

In international law, consent involves states, not individuals. Consent is a crucial principle of international law that necessitates the agreement of all relevant parties for any changes in rules to be legally binding. However, some legal scholars propose that a consensus among states, rather than the explicit consent of each state, may be the standard by which a rule is considered obligatory and enforceable.