epi

Epi, or epi-, is a combining form derived from the Greek prefix epi-, meaning upon, over, outside, or in addition to. It is used across many disciplines to form terms that indicate a relationship of position, progression, or relation to something else.

In biology and medicine, epi- terms describe location or layers of tissue or space near the body’s coverings. Examples include epidermis (the outer skin layer) and epithelium (the tissue that lines surfaces and forms glands). Epidural refers to the space around the dura mater in the spinal area, and epinephrine, the hormone also known as adrenaline, is used in emergency treatments. In genetics and development, epigenetics and the epigenome describe processes that regulate gene expression without changing the underlying DNA sequence.

In other fields, epi- appears in concepts such as epicenter, the point on the Earth’s surface directly above an earthquake’s focus, and epiphany, a sudden manifestation or insight. Epilogue, epigram, and epitaph are additional examples where epi- forms part of words conveying a sense of “upon” or “appearance” in their etymology.

Epi is also encountered as an acronym in various contexts. One common usage is the Environmental Performance Index (EPI), a composite measure used to compare national environmental performance. The term thus appears in scientific reports, policy discussions, and academic literature, often requiring context to determine whether epi- is serving as a prefix or an acronym.

See also: prefix epi-, epicenter, epigenetics, epinephrine, epidermis, epithelium.