What sort of disorder is failure to thrive in a child?

Could be a Problem. This is either a child whose physical growth-both height and weight are delayed well beyond age predicted values or a serious delay in emotional or intellectual development or both.This term describes a condition, but not the cause which needs looking into. Consult your pediatrician.
Lack of growth. Failure to thrive means a child is not growing as they should. There are charts doctors use to see how a child should be growing in height and weight. If a child stops growing or is not growing as fast as they should, this is often referred to as "failure to thrive".
Growth disorder. Failure to thrive (ftt) in a child can be a complex disorder involving growth retardation (weight and then height) requiring elaborate meticulous workup to rule out various disorders affecting various organ systems or it can be simply nutritional or even psychosocial. Md evaluation is definitely warranted.