MIDI Controllers are still designed and defined that way, but many digital pianos have now bridged that gap, being MIDI-capable and having inbuilt speakers that play the sound. The earliest MIDI Controller were made to be separate instruments with singular ability of transmitting MIDI data, but no ability to give out sounds on their own. And a MIDI Controller is any instrument or computer software that can transmit MIDI data to MIDI-compatible devices which are able to interpret them into corresponding musical parameters and produce sounds accordingly. MIDI is an acronym for Musical Instrument Digital Interface.
