Non-specific term indicating inappropriate weight loss or insufficient weight gain, due to insufficient caloric intake, insufficient caloric absorption, or excessive caloric demand.
Defined as weighing <5th%ile for age and sex, or weight deceleration crossing two major growth lines on a growth chart.
Does not imply a defect in social, intellectual, or emotional development, but if untreated may lead to any combination of those.
Classically divided into endogenous (medical) and exogenous (psychosocial or environmental) causes
Failure to thrive 
GERD
Stridor
IBS
Stridor 
Food allergy
Hiccups
Malabsorption