1. a flat surface determined by the position of three points in space.
2. a specified level, as the plane of anesthesia.
3. to rub away or abrade. See also planing and plastic surgery.
4. a superficial incision in the wall of a cavity or between tissue layers, especially in plastic surgery, made so that the precise point of entry into the cavity or between the layers can be determined.

• coronal p. — frontal plane, an ambiguous term when applied to quadrupeds and bipeds.
• dorsal p. — any plane passing longitudinally through the body from side to side, at right angles to the median plane and dividing the body into dorsal and ventral parts. Called also coronal plane, frontal plane.
• horizontal p. — one passing through the body at right angles to the median plane, and dividing the body into upper and lower parts.
• inclined p. — an intraoral acrylic or metal appliance used in orthodontics to guide a tooth into a new position by using pressure applied when the mouth is closed normally. Commonest use is to move canines laterally.
• median p. — one passing longitudinally through the body from front to back and dividing it into right and left halves.
• nasal p. — the space between the nostrils.
• nasolabial p. — the extension of the nasal plane between the nostrils into the upper lip in cattle.
• nuchal p. — the flat surface at the back of the occipital bone below the nuchal crest.
• rostral p. — the bare area on the dorsum of the snout of the pig.
• sagittal p. — a vertical plane through the body parallel to the median plane (or to the sagittal suture) and dividing the body into left and right portions.
• transverse p. — one passing through the body, at right angles to the sagittal and dorsal planes, and dividing the body into cranial and caudal portions.
• vertical p. — one perpendicular to a horizontal plane, dividing the body into left and right, or front and back portions.