Granuloma is a medical term for a tiny collection of immune cells known as macrophages. Granulomas form when the immune system attempts to wall off substances that it perceives as foreign but is unable to eliminate. Such substances include infectious organisms such as bacteria and fungi as well as other materials such as keratin and suture fragments. A granuloma is therefore a special type of inflammation that can occur in a wide variety of diseases . The adjective granulomatous means characterized by granulomas.

In medical practice, doctors occasionally use the term "granuloma" loosely to mean "a small nodule". Since a small nodule can represent anything from a harmless nevus to a malignant tumor, this usage of the term is not very specific. Examples of the inaccurate use of the term granuloma are the lesions known as vocal cord granuloma, pyogenic granuloma and intubation granuloma, all of which are examples of granulation tissue, not granulomas. "Pulmonary hyalinizing granuloma" is a lesion characterized by keloid-like fibrosis in the lung, and is not granulomatous. Similarly, radiologists often use the term granuloma when they see a calcified nodule on X-ray or CT scan of the chest. They make this assumption since granulomas often contain calcium, although the cells that form a granuloma are too tiny to be seen by a radiologist. The most accurate use of the term "granuloma" requires a pathologist to examine surgically removed and specially colored (stained) tissue under a microscope. The following is a more technical, accurate definition of a granuloma: In pathology, a granuloma (classical Greek plural granulomata; modern anglicized plural granulomas) is an organized collection of macrophages.