All terms in this list:

circuit: A revolving; A journey; A route.

circulatory: Of or pertaining to a circulation, especially to the circulatory system.

circumnavigate: To travel completely around somewhere or something, especially by sail.

circumstance: An event; a fact; a particular incident.

circumvent: to avoid or get around something; to bypass

period: A length of time.

periodic: happening at regular intervals; happening from time to time

perigee: The point in the orbit of any object which is at the smallest distance from the center of the central body. For example, the point in the moon's orbit which is the smallest distance from the center of the earth.

periphery: The outside boundary, parts or surface of something.

periscope: an instrument for seeing around things