Hermeneutics is the science of interpreting a written text. Rules of interpretation must be followed for each literary genre. The meaning of words is decided by the custom and the usage that existed in the author's time. For instance, you need to know the meanings of words as they were used at the time it was written, not in the current year, to properly understand the book of Revelation.
Reference: The Everything History of the Bible Book, Jeffery Donley, Ph.D., Adams Media.