Common words:
* Perimeter: The outer boundary of a two-dimensional shape.
* Periscope: A device used to see over an obstacle or from a concealed position.
* Perihelion: The point in a celestial object's orbit where it is closest to the sun.
* Pericardium: The sac that surrounds the heart.
* Perinatal: Relating to the period around the time of birth.
* Peripheral: Relating to the outer edge or boundary of something.
* Periodontal: Relating to the tissues surrounding the teeth.
* Peritonitis: Inflammation of the peritoneum (the lining of the abdominal cavity).
Less common words:
* Pericranium: The membrane that covers the outer surface of the skull.
* Periglacial: Relating to the region around a glacier.
* Periheliac: Relating to perihelion.
* Peripatetic: Traveling from place to place, especially working in different places for relatively short periods.
* Periplasm: The space between the plasma membrane and the outer membrane of a bacterium.
* Periphrasis: The use of more words than necessary to express an idea.
* Peristalsis: The wave-like muscular contractions that move food through the digestive tract.
* Peritrophic membrane: A thin membrane that surrounds the food bolus in the midgut of some insects.
* Periwig: A wig, especially a long, full wig worn by men in the 17th and 18th centuries.
* Periwinkle: A type of small, blue-purple flower.
Note: The prefix "peri-" usually means "around" or "surrounding."