Left-behind children refer to children whose parents or main guardians have left home for a long time because of work and life, which leads to the lack of necessary family education and care in the process of children's growth, and live with grandparents or other relatives.

The definition of left-behind children can be understood from the following aspects:

1. Parents or main guardians leave home for a long time: parents or main guardians of left-behind children leave home for a long time due to work and life needs and cannot accompany their children to grow up. The time away from home may be months, years or even longer.

2. Lack of necessary family education and care: Due to the lack of parents or main guardians, left-behind children lack necessary family education and care during their growth. They may not get the guidance and support of their parents, and are prone to psychological problems and behavioral deviations.

3. Living with grandparents or other relatives: In order to take care of left-behind children, their grandparents or other relatives may assume the responsibility of raising and educating. In this case, left-behind children may live with grandparents or other relatives, forming a new family environment.

4. Age range: The age range of left-behind children is not clearly defined, but it usually refers to preschool children and children in primary and secondary schools. This is because children at this stage need their parents' company and education more.

In short, left-behind children refer to children who live with grandparents or other relatives because their parents or main guardians are not at home for a long time, which leads to the lack of necessary family education and care during their growth. They are facing many difficulties and challenges and need the attention and support of the society.