There is many ways one can define Tangible User Interface (TUI), although all of them convey almost the same message. Below are the few of them:

• “Giving physical form to digital information that fits to our social, cultural and physical context”.

• “Computation that moves beyond the traditional confines of desktop & blends into our daily experiences of physical and digital world”.

• “Augmentation of Physical & Virtual world”.

• “Integrating information with Everyday Physical Objects”.

• “Mapping physical objects to digital data”.

Physical environment input becomes an interface to interact with digital information as subtle changes in color, movement, smell, form, sound, light, audio, video etc.

• Hiroshi Ishii and Brett Ulmer says,
“Tangible user interface = user interfaces that augment the real physical world by coupling digital information to everyday physical objects and environment.”

Few examples are mentioned below for appropriate understanding of above statements.

1. Marble Answering Machine:
The physical marbles represent incoming messages. The number of marbles that have moved into the pin-ball like chute indicates the number of messages. When a marble is dropped into a slot in the machine, the recorded message is played. Dropping the same marble into another slot on the phone dials the caller who left the message.

2. Urban Planning Workbench (Urp):
Urban planning workbench uses scaled physical models of architectural buildings to configure and control an underlying urban simulation of shadow, light reflection, wind flow, etc, based on the positions and orientations of physical models of buildings, on the table surface.

3. Music Bottles:
Music bottles project is a designed table and various sets of three corked bottles that "contain" the sounds of three instruments or tracks of various musical genres. When a bottle is placed onto the stage area of the table and the cork is removed, the corresponding instrument becomes audible.

Tangible interaction is an interdisciplinary area. It includes a span of several disciplines such as HCI, interaction design, product/industrial design, ubiquitous computing, electronics and computer science.