A keyboard that is arranged such that a given chord can be transposed to any key without changing the fingering. This is not possible on a conventional piano keyboard because, as the chord is played in different keys, it will be necessary to rearrange the fingers to accommodate the pattern of black and white keys for that chord in that particular key.
An example from the 19th century is the von Janko keyboard, invented in 1882. It has six rows of keys, in two alternating patterns. Each row has six keys per octave, with the interval between adjacent keys being a whole step apart. By moving up or down a row, it is always possible to move a chord to a different key without alterning the fingering. No example of the von Janko keyboard is currently in production.
A contemporary example is the C-Thru Music Axis-64 keyboard. This takes a different approach to the problem. Its keyboard does not remotely resemble a conventional piano keyboard; rather, its keys are in the form of hexogonal tiles, arranged in a matrix. A chord may be transposed by moving it up, down, or across the keyboard. The Axis-64 serves as a master keyboard; it sends MIDI note messages in a conventional fashion. A smaller and lower cost version, the Axis-49, is also available. (Note: As of January 2015, C-Thru Music is no longer in business.)