Metaphors are figures of speech where one things is said to be something else to suggests a comparison.
A 'dead metaphor' is a term that has become so frequently used that is has become understood to literally – rather than figuratively – refer. This is one way language changes over time.
For example, in astronomy references to the 'life cycles', 'births' and 'deaths' of inanimate stars are now used as if technical terms and not just figures of speech.
Read 'The passing of stars: Birth, death, and afterlife in the universe'
