Definition: The philtrum is the indentation that runs from the bottom of the nose to the center of the upper lip. Someone with a pronounced philtrum might be described as having a "cupid's bow" mouth.
Children with fetal alcohol syndrome (FAS) are often described as having a flat philtrum. There is little or no indentation, and their upper lip lacks a "dip" in the middle. For more about Fetal Alcohol Syndrome and to find online resources to help you parent a child with this disability, read "Before You Look for Information on Fetal Alcohol Spectrum Disorder" and consult the books I've reviewed about FASD. You can find a diagram and list of facial features associated with FASD on About.com's Alcoholism site.
You may also hear philtrum referred to if your child has a cleft lip. For information on surgery for cleft lip, including reconstruction of the philtrum, read the articles on cleft lip repair on About.com's Plastic Surgery site.