Damage is any change in a thing, often a physical object, that degrades it away from its initial state. It can broadly be defined as "changes introduced into a system that adversely affect its current or future performance". Damage "does not necessarily imply total loss of system functionality, but rather that the system is no longer operating in its optimal manner". Damage to physical objects is "the progressive physical process by which they break" and includes mechanical stress that weakens a structure, even if this is not visible. Efforts undertaken to prevent or ameliorate damage are often referred to as "damage control".

All physical damage begins on the atomic level, with the shifting or breaking of atomic bonds, and the rate at which damage to any physical thing occurs is therefore largely dependent on the elasticity of such bonds in the material being subjected to stress. Damage can occur where atomic bonds are not completely broken but are shifted to create unstable pockets of concentration and diffusion of the material, which are more susceptible to later breakage. The effect of outside forces on a material depends on the relative elasticity or plasticity of the material; if a material tends towards elasticity, then changes to its consistency are reversible, and it can bounce back from potential damage. However, if the material tends towards plasticity, then such changes are permanent, and each such change increases the possibility of a crack or fault appearing in the material.

Although all damage at the atomic level manifests as broken atomic bonds, the manifestation of damage at the macroscopic level depends on the material and can include cracks and deformation, as well as structural weakening that is not visible.

Damage to structures and other objects can take several forms, such as fire damage caused by the effects of burning, water damage done by water to materials not resistant to its effects, and radiation damage due to ionizing radiation. Some kinds of damage are specific to vehicles and mechanical or electronic systems, such as foreign object damage caused by the presence of any foreign substance, debris, or article; hydrogen damage caused by interactions between metals and hydrogen; and damage mechanics, which includes damage to materials due to cyclic mechanical loads. When an object has been damaged, it may be possible to repair the object, thereby restoring it to its original condition or to a new condition that allows it to function despite the damage.

Damage can be intentional or accidental. When an object or building is deliberately damaged, the act can be socially significant as a way to degrade the meaning of the object. Though things can have multiple meanings, in circumstances where they are deliberately damaged one meaning—which prompts the act—takes precedence.

Damage to a living organism may be referred to as an injury. The kinds of damage experienced by living things range from microscopic DNA damage and indirect DNA damage, and cell damage in a biological cell, to damage to larger systems such as nerve damage, brain damage, and articular cartilage damage. Damage to the body on a larger scale is often referred to as trauma. Ageing is accompanied by a loss of ability to recover quickly from various kinds of biological damage. Damage is one of two factors proposed to influence biological ageing (the other being programmed factors follow a biological timetable). Damage-related factors include internal and environmental assaults to living organisms that induce cumulative damage at various levels.

Damage that occurs as an unintended consequence of an effort to cause intentional damage elsewhere, such as with a military operation, is called collateral damage, while damage specifically done to public or private property is called property damage. By extension, damage is also used to describe a degradation in the value of intangible things such as relationships, self-image, reputation, and goodwill.

The propensity for damage to occur to physical objects and systems, as well as to intangible characteristics, is built into the prices of goods and services that depend on the supply of these things, particularly as a component of insurance costs. In common law, damages are the award, typically of money, to be paid to a person as compensation for loss or injury (i.e., economic or physical damage).