Evil is a term describing that which is regarded as morally bad, intrinsically corrupt, wantonly destructive, inhumane, or wicked. In most cultures, the word is used to describe acts, thoughts, and ideas which are thought to (either directly or causally) bring about affliction and death — the opposite of life. However, the definition of what counts as evil differs widely from culture to culture and from individual to individual. Some philosophers reject the idea of evil. Plato, for example, argued that that which we call evil is merely ignorance, and that which we call good is merely that which everyone desires.

For those who accept the existence of evil, there are two main beliefs about evil ^{[citation needed]}. In some belief systems, evil consists of a willful deviation from a code of laws (written or unwritten) or moral standard, usually proscribed by a holy deity. According to this definition, people who, for example, reject a certain belief or engage in sexual practices against this code are engaged in evil acts. According to other belief systems, evil consists of intentionally doing harm, and so-called "victimless crimes" should not be considered evil.

The duality of 'good versus evil' is expressed, in some form or another, by many cultures. This concept describes a hierarchy of moral standards applied to human behaviour. In more casual or derogatory use, the word "evil" can characterize people and behaviours that are hurtful, ruinous, or disastrous. Those who believe in the duality theory of evil believe that evil cannot exist without good, nor good without evil, as they are both objective states and opposite ends of the same scale.

A similar term, malice, (from the Latin malus meaning "bad") describes the deliberate human intent to harm and be harmful. "Evil", by contrast, tends to represent a more elemental concept; a disembodied spirit that is natural and yet abominable. Whereas "malice" is specifically concerned with the act itself, "evil" is the cause of a malicious act. True evil is sometimes motivated by malicious greed and/or sadism.