a language of instruction information, designed to allow pieces of musical equipment to communicate with one another. MIDI information is information about action to be taken (for example the instruction to play a specific note) and not that action itself. (In this way MIDI information might be considered as similar to written notation. It is instruction to make sound and not the sound itself (a synthesiser or instrumentalist must be able to receive the instructions and to follow them out to make the sound).
MIDI is an acronym so should always be written with capital letters. It stands for Musical Instrument Digital Interface.