The term 'AGE UNIT' can be solved by the crossword solution 'YEAR', which is a four-letter word. A year is a unit of time that is commonly used to measure the age of something or someone. It is typically defined as 365 days, although in some cases it can be 366 days. The Gregorian calendar is the most widely used calendar system in the world and it divides the year into 12 months. Each month is divided into weeks, and each week is divided into days. The length of a year is determined by the Earth's orbit around the Sun, and it is used to measure the passage of time.