Chording means pushing several keys or buttons simultaneously to achieve a result.
In music, more than one key are pressed at a time to achieve more complex sounds, or chords.
Chording, with a chorded keyboard or keyer allows one to produce as many characters, as a QWERTY keyboard, but with fewer keys and less motion per finger, and is thought to alleviate or prevent carpal tunnel syndrome.
Mouse chording allows a user to use a two-button mouse, trackball, or touchpad as if it where a three-button device. For example, in the Unix graphical user interface (X11), the middle button is used to paste text. Since Microsoft-type mice traditionally only had two buttons, users of Unix-type systems such as Linux and BSD chord the right and left buttons to paste text.
TipTapSpeech an application for the iPhone and iPad is a chord-based text entry solution for touch screen computing.
A GKOS chording keyboard application development for iPhone was started on the GKOS Google Group  on May 25, 2009. The application for iPhone became available on May 8, 2010, and a similar application for Android on October 3, 2010. Thumbs are used to press the keys that are located towards the sides of the screen, either a single key or two keys simultaneously.