健康是指生物的功能性和代谢效率的水平。对人类而言，健康是指个人或群体面临生理、心理或社会的挑战时，适应及自我管理的能力。世界卫生组织于1946年定义健康广义来说是：「健康不仅为疾病或虚弱之消除，而是体格，精神与社会之完全健康状态。」此定义相当有争议，特别在其实务上不易评估，以及用到「完全」一词所引发的相关问题。后来也有组织提出新的定义，有些定义将健康及个人的满足相关连。例如国际健康功能与身心障碍分类系统（ICF）及国际疾病与相关健康问题统计分类（ICD）等国际分类系统，也常用来定义及衡量健康的组成。

Health is the level of functional or metabolic efficiency of a living organism. In humans it is the ability of individuals or communities to adapt and self-manage when facing physical, mental or social challenges. The World Health Organization (WHO) defined health in its broader sense in its 1948 constitution as "a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity." This definition has been subject to controversy, in particular as lacking operational value and because of the problem created by use of the word "complete" Other definitions have been proposed, among which a recent definition that correlates health and personal satisfaction. Classification systems such as the WHO Family of International Classifications, including the International Classification of Functioning, Disability and Health (ICF) and the International Classification of Diseases (ICD), are commonly used to define and measure the components of health.