[Greek peri, about, around.]
Many examples were formed in Greek and have reached English through Latin and French, such as peripatetic, travelling from place to place (Greek peripatētikos, walking up and down), or periphery (Greek periphereia, circumference, from pherein, to bear).
The form is common in modern scientific and medical terms. A pericarp (Green karpos, fruit) is the part of a fruit formed from the wall of the ripened ovary; a periglacial area is one adjacent to a glacier or ice sheet or otherwise subject to repeated freezing and thawing; the pericardium (Greek kardia, heart) is the membrane enclosing the heart; periodontics (Greek odous, odont-, tooth) is the branch of dentistry concerned with the structures surrounding and supporting the teeth; a periurban area is countryside immediately adjacent to a built-up area.
In astronomy, peri- refers to the point in the orbit of a celestial object or spacecraft at which it is nearest to its parent body, so perihelion (Greek hēlios, sun), the point in an orbit closest to the sun; perilune (Latin luna, moon), the point at which a spacecraft in lunar orbit is closest to the moon; perijove, similarly the point in its orbit around Jupiter at which a satellite or spacecraft is closest to it. Its opposite is apo-.