An MIA mom is "missing in action." She is totally unavailable for her kid or teachers. MIA moms are not there on the first day of school, back to school night, open house, and can't be reached for conferences.
Marissa's mother is an MIA mom. We've called home three times and the Vice Principal even went to her apartment to try to find her. Actually, we don't know if Marissa even has a mom. Related to helicopter mom, SCUD parent.