Underpitching is the opposite of overpitching. Underpitching is when too little yeast is added to the wort. This is a result of there being too little viable yeast in the starter/yeast pack or the batch size being too large for the amount of yeast used (ie pitching a Wyeast smack pack into a 10 gallon batch). The following can occur as a result of underpitching-
1- The yeast can become stressed and produce off flavours and aromas
2- There may be a large lag time which increases the chances of an infection.