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Meaning of fourth dimension

fourth' dimen'sion

Pronunciation: [key]
  1. a dimension in addition to length, width, and depth, used so as to be able to employ geometrical language in discussing phenomena that depend on four variables: Time is considered a fourth dimension for locating points in space-time.
  2. something beyond the kind of normal human experience that can be explained scientifically: The story deals with ESP and other excursions into the fourth dimension.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.