Radiation Something that exists, apparently to transfer energy, since the beginning of time.
Random Something that is not predictable. Theoretical more than real. It is difficult to create randomness, but seems to frequently occur in nature.
Red A color. Usually associated with bright and bold. Has mostly negative connotations, i.e. stop sign, prostitution, attract bulls.
Riddle A method in language to drive others nuts.
Riot A usually violent act of civil disobedience performed by a group of people who feel that some injustice is occurring that can be and needs to be corrected. The injustice can range from racial profiling to a bad call by a referee in a soccer game. Or any unorganized rabble that is destructive.
Robot A mechanical replacement for something. Built, not birthed.
Roman That which is of Rome, an ancient city in Italy. Once the capitol of an empire.
Roman Catholic A large sect of the Christian religion Catholicism. Usually conservative to the tenants of the religion.
Rome A large ancient city in a European country that is now called Italy.
Rubber One of those incredible substances that is constantly renewable, and can be very hard to get rid of.
Rubik's Cube A "toy" in the shape of a cube with nine squares on each side, each side a different color. There is a pivot mechanism in the middle of the cube that allows each face to turn, thus mixing up the colors. The game is to mix up the colors then put them back in order.