The term VAST SPAN can be broken down into two crossword solutions, AEON (4 letters) and EON (3 letters). AEON is a noun meaning an indefinitely long period of time, while EON is a noun meaning an immeasurably or indefinitely long period of time. Both words are derived from the Greek word aion, which means 'AGE' or 'FOREVER'. The term VAST SPAN therefore refers to a very long period of time, one that is almost impossible to measure.