In metaphysics, the word individual, while sometimes meaning "a person", more typically describes any numerically singular thing. Used in many contexts, both 'Socrates' and 'the Moon' denote individuals; 'grapefruit' and 'redness' (at least often) do not. 'Individual' as a piece of philosophical jargon is much-bandied and often to be found in the company of particular -- indeed, often treated as synonymous with 'particular' (though one wonders if abstract particulars can count as individuals) -- and contrasted with 'universal'.
A famous work on individuals and their individuation is by P. F. Strawson: Individuals: An Essay in Descriptive Metaphysics (London: Methuen & Co. Ltd., 1959; New York: Anchor, 1963).
In everyday life an individual is some collection of thoughts and deeds that is considered an entity. Many people consider an individual responsible for its actions.
What collection of thoughts and deeds is considered an individual depends on perspective. For example, the thoughts and deeds of one body may be considered one individual.
However, it happens that after accidents and illnesses that caused brain damage, the thoughts and deeds from one body become so drastically different that many people don't feel that that body holds the same individual.
Some people talk about elderly people or handicapped or ill or poor people without the slightest regard of the risk that they may meet the same fate. They are right about that if they have a time-limited perspective on individuality. They consider the elderly, handicapped, ill or poor individual that may later occupy their body to be someone else.
In the past, persons were not considered responsible for the damage they caused while drunk. Apparently, in the drunk state, the body was considered not to hold the same individual as in the sober state.
One may assume that momentary madness can happen to everyone (something like a computer crash). The normal individual is considered absent from the body during that state of madness, and therefore not responsible. (Compare with temporary insanity and automatism).
For example, in 1795 in England, a young woman who had taken good care of her ill mother for many years, suddenly killed her. It was considered that a momentary madness was caused by severe fatigue from her hard work. Especially the days before the awful event had been very hard, because of her mother's condition. The woman was not prosecuted. She later married and had children and never committed any crime.
In 1999 in the Netherlands, a young mother put her baby in the microwave instead of the baby's milk. The death of the baby was considered to be caused by a very unfortunate blackout. The mother was considered absent from the body when it happened. She was not prosecuted, but comforted for her loss.