Rod Serling thought he had come up with the term "Twilight Zone" on his own (he liked the sound of it), but after the show aired he found out that it is an actual term used by Air Force pilots when crossing the day / night sides above the world.
Series creator Rod Serling made up the phrase "Sixth Dimension" to use in season one's opening narration. William Self of CBS-TV asked him what was the fifth dimension (given that dimensions one through three are exemplified by a line, a plane, and a cube, respectively, and the fourth is time). Serling answered, "I don't know. Aren't there five?" He then changed the narration to "There is a fifth dimension..."