Skeuomorphism refers to a design principle in which design cues are taken from the physical world. This term is most frequently applied to user interfaces (UIs), where much of the design has traditionally aimed to recall the real world - such as the use of folder and files images for computer filing systems, or a letter symbol for email - probably to make computers feel more familiar to users. However, this approach is increasingly being criticized for its lack of ingenuity and its failure to pioneer designs that truly harness a computer's superior capabilities, rather than forcing it to merely mimic the behavior of a physical object.
The term skeuomorphism is derived from the Greek words "skeuos," which means vessel or tool, and "morphe," which means "shape."
Skeuomorphism has famously been one of Apple's key design principles, and part of its Human Interface Guidelines. However, the form of skeuomorphism Apple espouses has largely been a subtle form that suggests something real, but doesn't necessarily attempt to replicate it. However, in 2011, Apple came under fire from users when some of its iOS applications took on a decidedly country-western flavor.
Overall, skeuomorphism has increasingly come under fire, largely because many of the nostalgic elements it attempts to portray - such as calendars, day planners, address books, etc. - are almost entirely foreign to younger generations of users. In addition, critics of skeuomorphism point to this reliance of physical objects in design as an impediment to making more useful designs. For example, many digital calendars look and behave much like a regular paper wall calendar; dismissing this structure could make them a lot more intuitive for users. In other words, design can be constrained by being bound to physical objects, even though computers are not subject to those constraints.