Q.E.D. is an abbreviation for the Latin phrase 'quod erat demonstrandum, ' which translates to 'which was to be demonstrated.' It is used to signify the end of a mathematical proof or argument, and is often written at the end of a statement to indicate that the proof has been completed. The crossword solution ERAT is an anagram of the word RATE, which is related to the idea of demonstrating something.