Seventy is a number that comes after sixty-nine and before seventy-one.
When we use the word "seventy", it's usually when we want to talk about ages, prices, or quantities. For example, if someone is seventy years old, it means they were born around the time of World War II. If something costs seventy dollars, it's priced at a certain amount that's higher than sixty-nine and lower than seventy-one dollars. We also use "seventy" to describe groups of people or things - for instance, if there are seventy students in your class, it means you have many classmates.
The decade of the 1870s, 1970s, etc.
From or evoking the 71st through 80th years of a century (chiefly the 1970s).
Example: I know the theme of the school dance is "The 1970s" but is covering the walls, floor, and ceiling with tie-dye too seventies?
