An enemy or a foe is an individual or a group that is considered as forcefully adverse or threatening. The concept of an enemy has been observed to be "basic for both individuals and communities". The term "enemy" serves the social function of designating a particular entity as a threat, thereby invoking an intense emotional response to that entity. The state of being or having an enemy is enmity, foehood or foeship.

Enemy comes from the 9th century Latin word inimi, derived from Latin for "bad friend" (Latin: inimicus) through French. "Enemy" is a strong word, and "emotions associated with the enemy would include anger, hatred, frustration, envy, jealousy, fear, distrust, and possibly grudging respect". As a political concept, an enemy is likely to be met with hate, violence, battle and war. The opposite of an enemy is a friend or ally. Because the term "the enemy" is a bit bellicose and militaristic to use in polite society, informal substitutes are more often used. Often the substituted terms become pejoratives in the context that they are used. In any case, the designation of an "enemy" exists solely to denote the status of a particular group of people as a threat, and to propagate this designation within the local context. Substituted terms for an enemy often go further to meaningfully identify a known group as an enemy, and to pejoratively frame that identification. A government may seek to represent a person or group as a threat to the public good by designating that person or group to be a public enemy, or an enemy of the people.

The characterization of an individual or/and group as an enemy is called demonization. The propagation of demonization is a major aspect of propaganda. An "enemy" may also be conceptual; used to describe impersonal phenomena such disease, and a host of other things. In theology, "the Enemy" is typically reserved to represent an evil deity, devil or a demon. For example, "in early Iroquois legend, the Sun and Moon, as god and goddess of Day and Night, had already acquired the characters of the great friend and enemy of man, the Good and Evil Deity". Conversely, some religions describe a monotheistic God as an enemy; for example, in 1 Samuel 28:16, the spirit of Samuel tells a disobedient Saul: "Wherefore then dost thou ask of me, seeing the LORD is departed from thee, and is become thine enemy?"

"The enemy," as the object of social anger or repulsion, has throughout history been used as the prototypical propaganda tool to focus the fear and anxiety within a society toward a particular target. The target is often general, as with an ethnic group or race of people, or it can also be a conceptual target, as with an ideology which characterizes a particular group. In some cases the concept of the enemy have morphed; whereas once racial and ethnic claims to support a call to war may later have changed to ideological and conceptual based claims.

During the Cold War, the terms "Communists" or "Reds" were believed by many in American society to mean "the enemy," and the meaning of the two terms could be extremely pejorative, depending on the political context, mood, or state of fear and agitation within the society at the time.

There are many terms and phrases that allude to overlooking or failing to notice an enemy, such as Trojan horse or wolf in sheep's clothing. Generally, the counterpoint to an enemy is a friend or ally, although the term frenemy has been coined to capture the sense of a relationship wherein the parties are allied for some purposes and at odds with one another for other purposes.

The existence or perceived existence of a collective enemy tends to increase the cohesiveness of the group. However, the identification and treatment of other entities as enemies may be irrational, and a sign of a psychological dysfunction. For example, group polarization may devolve into groupthink, which may lead members of the "in" group to perceive nonmembers or other groups as enemies even where the others present neither antagonism nor an actual threat. Paranoid schizophrenia is characterized by the irrational belief that other people, ranging from family members and personal acquaintances to celebrities seen on television, are personal enemies plotting harm to the sufferer. Irrational approaches may extend to treating impersonal phenomena not merely as conceptual enemies, but as sentient actors intentionally bringing strife to the sufferer.

The concept of the enemy is well covered in the field of peace and conflict studies, which is available as a major at many major universities. In peace studies, enemies are those entities who are perceived as frustrating or preventing achievement of a goal. The enemy may not even know they are being regarded as such, since the concept is one-sided.