Final answer:
The term for fluid, infection, or other matter encapsulated in a firm mass under the skin is an abscess. Abscesses are encapsulated sacs filled with pus and signify an infection that the body has cordoned off, often needing medical treatment.
Step-by-step explanation:
The term for fluid, infection, or other matter encapsulated in a firm mass under the skin is an abscess. An abscess is typically an encapsulated sac filled with fluid, semi-solid matter, or gas that is usually located just below the upper layers of skin. These lesions can be pus-producing; in medical terms, they are described as suppurative. An abscess can arise from any suppurative (pus-producing) infection of the skin, which can include bacterial infections such as those that cause acne. Clinically, abscesses are of concern because they signify an infection that the body has attempted to wall off; they can be painful and may require medical intervention.
While there are other terms like cyst, tumor, and hematoma, each of these has distinct characteristics. A cyst is a closed pocket of tissue that can be filled with air, fluid, pus, or other material; a tumor refers to any abnormal swelling, lump, or mass which may or may not be cancerous; and a hematoma is a localized collection of blood outside blood vessels, usually in liquid form within the tissue.