The term 'Period of Power' refers to a period of time in which a certain individual or group is in control. The crossword solutions for this term are 'REIGN' (5 letters) and 'DAY' (3 letters). REIGN is a synonym for rule or government, and DAY is a unit of time. Therefore, the term 'Period of Power' can be interpreted as a period of time in which a certain individual or group is in control of a government or rule.