What is age-specific enrolment ratio? ASER?

Age-specific Enrolment Ratio (ASER), is calculated wither for a single-age or age-group children and consider the total enrolment of that age or age group irrespective of the grade or level of Education in which a child is enrolled which is then divided by single-age/age-group child population and multiplied by 100 to obtain ASER.