身体的疼痛指通常由身体损伤、病患或不良的外部刺激所引起的不舒服感觉。出于临床研究的需要，国际疼痛研究协会（International Association for the Study of Pain）将疼痛定义为「由真正存在或潜在的身体组织损伤所引起的不舒服知觉和心理感觉」

疼痛是主观的感觉，早于1968年疼痛处理专家Margo McCaffery首次提出一个在护理学界普遍使用的定义：「一个人说感到痛，这就是痛；他说痛仍在，痛就仍在。」（原文：Pain is whatever the experiencing person says it is, existing whenever he says it does）

Pain is a distressing feeling often caused by intense or damaging stimuli, such as stubbing a toe, burning a finger, putting alcohol on a cut, and bumping the "funny bone". Because it is a complex, subjective phenomenon, defining pain has been a challenge. The International Association for the Study of Pain's widely used definition states: "Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage." In medical diagnosis, pain is a symptom.