Skeumorphism is when a user interface is stylized to look like real world materials (e.g. the Calendar.app on OSX is made to look like a leather desk calendar).

Skeumorphism is when a user interface is stylized to look like real world materials (e.g. the Calendar.app on OSX is made to look like a leather desk calendar). The big argument toward it is that it either looks tacky or ruins usability.