The term 'Historical Span' refers to the length of time that a particular event or period of history has been in existence. The crossword solutions DECADE (6 letters), AGE (3 letters) and ERA (3 letters) all refer to different lengths of time. A DECADE is a period of 10 years, an AGE is a period of 20 years and an ERA is a period of 100 years. Therefore, the term 'Historical Span' can refer to any of these three lengths of time.