organ

An organ is an anatomical structure that performs a specific function or group of related functions as an integrated unit. Organs are typically composed of two or more tissue types (epithelial, connective, muscular, and/or nervous tissue) that work together. The heart, liver, kidney, lung, and skin are classic examples, but the classification can extend to smaller structures like lymph nodes or the adrenal gland, which similarly integrate multiple tissue types for a dedicated function.

Organs are the building blocks of organ systems — groups of organs that cooperate to perform broader physiological functions. For example, the heart and blood vessels together form the cardiovascular system; the stomach, intestines, liver, and pancreas together form the digestive system. Individual organs have their own intrinsic functions but operate within the context of their system, and dysfunction in one organ affects the entire system's performance.

Yes — the skin is the largest organ of the human body by surface area (~1.7–2 m² in adults). It qualifies as an organ because it integrates multiple tissue types (epidermis, dermis, hypodermis, plus hair follicles, sweat glands, and sensory receptors) performing diverse functions: protection against pathogens and UV radiation, temperature regulation, sensory detection, vitamin D synthesis, and water loss prevention. This multitissue, multifunctional integration is the hallmark of organ status.