身体的疼痛（pain）是由疾病、创伤、外在伤害性刺激，伴随现有的或潜在的组织损伤，所引起在知觉上或情绪上难受的、不愉快的感觉。疼痛是主观性的，影响的范围可为身体局部或整体，属一种复杂的生理、心理活动。
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出于临床研究的需要，国际疼痛研究国际疼痛研究协会将疼痛定义为“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”。[1][2]
疼痛是主观的感觉，早于1968年疼痛处理专家马戈·麦加费利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.）[3][4]
身体疼痛是看医生的主要原因；在美国，有一半看医生的人是因为疼痛，是最普遍的原因。[5]疼痛也是大部分病患的征状，可根据维持时间、强度、种类（如火烧、针刺感等）、位置等来诊断。通常疼痛会自然消退，或通过简单的止痛处理后消退，这可称作“急性”疼痛。但有时疼痛本身已是一种病患，如慢性疼痛。近年痛症引起了不同学科专家的关注，如药理学、神经生物学、护理学、物理治疗和心理学，并形成了疼痛管理这门在麻醉学、物理治疗、神经学、精神科等专科之下的附属专科[6]。
人体的痛觉神经有六种[需明示出处]，但科学上纪录有人无法感受痛楚，这种病是先天性无痛症，英文简称为“CIP”。
汉语中，痛与疼是近义词；在语意上，痛的含义较广泛，痛包含疼，且痛大于疼。“痛”较偏向慢性的、长久持续的、深入的、钝挫的疼痛。“疼”则较偏向急性的、短暂间断的、浅表的、热灼的、开放发散的、尖锐的疼痛[7]。
身体的“疼痛”，是主观、有意识的感觉。而“痛觉”，则指感受器、周围神经系统、脊髓和脑受有害刺激而引起的无意识活动[8]。一般而言，酸、痛、乏的症状与肌肉、软组织损伤较有关，而麻、抽、胀的感受则多和神经病变较相关；神经痛及神经性疼痛则为例外情形。
痛觉感受器受到化学、热力或撞击等可损伤身体组织的刺激就可能会产生疼痛。若神经系统由病患或损伤而受损，可引致神经痛症或神经性病变痛症[9]。由感受器受刺激和神经系统受损引起的疼痛是两大主要疼痛发生机制，第三类是精神性疼痛，较为罕见。
其中，痛觉感受器引起的疼痛可细分为三种：[10]
- 表面躯体疼痛（或皮肤疼痛）由皮肤或身体表面组织受损而引起。由于皮肤痛觉感受器分布细密，所以其产生的痛觉明显、位置明确但短暂。小伤口和轻度烧伤引起的疼痛属此类。
- 深层躯体疼痛源自韧带、腱、骨、血管或肌肉，由躯体痛觉感受器感应，其分布较疏，引起隐隐作痛的感觉，位置亦不明显。扭伤、断骨和肌膜疼痛症候群属此类。
- 内脏疼痛源自身体的器官。内脏痛觉感受器的分布更疏，产生的痛感更不易定位，且可转化为体腔壁痛、牵涉痛。
疼痛是许多疾病的常见或主要症状，如脑肿瘤的头痛、冠心病发作时的胸痛，胆石症的胆绞痛和腹痛、晚期癌症患者的癌性疼痛等。有些疼痛本身就是一种疾病，如带状疱疹的神经痛、三叉神经痛。疼痛不仅给患者带来痛苦，而且还对人体中枢神经、循环、呼吸、内分泌、消化和自主神经等系统造成不良影响，甚至是某些严重的、威胁生命的疾病症状。[11]
This often quoted definition was first published in 1979 by IASP in Pain journal, number 6, page 250. It is derived from a definition of pain given earlier by Harold Merskey: "An unpleasant experience that we primarily associate with tissue damage or describe in terms of tissue damage or both." Merskey, H. (1964), An Investigation of Pain in Psychological Illness, DM Thesis, Oxford.
See IASP Pain Terminology 互联网档案馆的存档，存档日期2008-05-12.. The whole entry on the term pain itself reads like this:
Pain. An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. Note: The inability to communicate verbally does not negate the possibility that an individual is experiencing pain and is in need of appropriate pain-relieving treatment. Pain is always subjective. Each individual learns the application of the word through experiences related to injury in early life. Biologists recognize that those stimuli which cause pain are liable to damage tissue. Accordingly, pain is that experience we associate with actual or potential tissue damage. It is unquestionably a sensation in a part or parts of the body, but it is also always unpleasant and therefore also an emotional experience. Experiences which resemble pain but are not unpleasant, e.g., pricking, should not be called pain. Unpleasant abnormal experiences (dysesthesias) may also be pain but are not necessarily so because, subjectively, they may not have the usual sensory qualities of pain. Many people report pain in the absence of tissue damage or any likely pathophysiological cause; usually this happens for psychological reasons. There is usually no way to distinguish their experience from that due to tissue damage if we take the subjective report. If they regard their experience as pain and if they report it in the same ways as pain caused by tissue damage, it should be accepted as pain. This definition avoids tying pain to the stimulus. Activity induced in the nociceptor and nociceptive pathways by a noxious stimulus is not pain, which is always a psychological state, even though we may well appreciate that pain most often has a proximate physical cause.
McCaffery M. Nursing practice theories related to cognition, bodily pain, and man-environment interactions. LosAngeles: UCLA Students Store. 1968.
More recently, McCaffery defined pain as "whatever the experiencing person says it is, existing whenever the experiencing person says it does.” Pasero, Chris; McCaffery, Margo. Pain: clinical manual. St. Louis: Mosby. 1999. ISBN 0-8151-5609-X.. From the American Board of Medical Specialties 互联网档案馆的存档，存档日期2008-05-16. website: "Pain Medicine is the medical discipline concerned with the diagnosis and treatment of the entire range of painful disorders. (...) Due to the vast scope of the field, Pain Medicine is a multidisciplinary subspecialty (...)." "Activity induced in the nociceptor and nociceptive pathways by a noxious stimulus is not pain, which is always a psychological state, even though we may well appreciate that pain most often has a proximate physical cause." Source: IASP Pain Terminology 互联网档案馆的存档，存档日期2008-05-12.. Compare definitions at IASP Pain Terminology 互联网档案馆的存档，存档日期2008-05-12.: "Neurophathic pain — Pain initiated or caused by a primary lesion or dysfunction in the nervous system." and "Neurogenic pain — Pain initiated or caused by a primary lesion, dysfunction, or transitory perturbation in the peripheral or central nervous system."
