An organ is a distinct structure made of multiple tissue types that work together as a single functional unit to perform a specific job in a living organism. In the body’s hierarchy, an organ sits between tissues (simpler) and organ systems (more complex).

Core idea: what makes an organ an organ

To count as an organ, a structure usually:

  • Is made of more than one type of tissue (for example, muscle, connective, nerve, and epithelial tissue together).
  • Forms a single functional unit with a clear, relatively specific job (pumping blood, filtering waste, exchanging gases, digesting food, etc.).
  • Is structurally distinct in the body (it has a recognizable shape, boundaries, and location, like the heart in the chest or the kidney in the abdomen).
  • Fits into a larger organ system , working together with other organs to carry out a broader body function (e.g., heart and blood vessels in the cardiovascular system).

Put simply, cells form tissues, tissues combine into organs, and organs cooperate in organ systems.

Formal definitions (biology and medicine)

Biology and medical references describe an organ with very similar wording:

  • A collection of tissues organized into a structural unit that serves a common or particular function.
  • A differentiated structure (heart, kidney, leaf, stem, etc.) made of cells and tissues and performing some specific function in an organism.
  • In medicine, a part of the body made of cells and tissues that perform a specific function , such as the heart, lungs, stomach, liver, kidney, skin, spleen, uterus, or ovary.

These definitions all stress specialization (a particular function) and multi- tissue composition.

Examples that show the criteria

  • Heart : Mostly cardiac muscle tissue, but also connective tissue, nervous tissue, and blood vessels; all together, they pump blood.
  • Stomach : Epithelial tissue lining the inside, smooth muscle in the wall, connective and nerve tissue; together they churn and chemically digest food.
  • Lungs : Airway epithelium, elastic connective tissue, blood vessels, and supporting structures that enable gas exchange.

Each example shows multiple tissue types, a clear boundary in the body, and one main job.

Where is the line? (borderline and broader uses)

The term “organ” can be used in a few overlapping ways:

  • Classic organs : Heart, liver, kidneys, lungs, brain, skin, etc.
  • Functional organ groups : Sometimes people talk about “visual organs” (eyes plus supporting structures) or “reproductive organs,” which are sets of related parts cooperating in an activity.
  • Non-animal organs : In plants, leaves, roots, stems, and flowers are also called organs because they are distinct structures performing specific functions like photosynthesis or support.

Disagreement can arise at the edges (for example, is a small gland its own organ or part of another organ?), but the same criteria—multiple tissues, distinct structure, specific function—are what biologists use to decide.

TL;DR: What makes an organ an organ is not just its importance, but the fact that it is a structurally distinct body part built from multiple tissues that together carry out a particular function within an organism and slot into a larger organ system.

Information gathered from public forums or data available on the internet and portrayed here.