A provision of the MIDI message format that allows certain consecutive messages of the same type to be combined, in order to reduce the amount of data sent. For example, the note on message contains three bytes: a command byte which indicates the message type and the channel number, a note number byte, and a velocity byte. However, if the sender is sending two note on messages in a row, both on the same channel, then for the second one the command byte need not be sent again; the velocity byte for the first note on may be followed immediately by the note number byte for the second one. Running status may continue indefinitely; the receiver knows that running status has ended when it receives a new command byte. However, a real time message is allowed to interrupt running status without ending it.
As one can see, this can get rather complicated. Running status is not well understood by all MIDI implementors, and it is still not uncommon to find synths or software which have bugs in the way they handle running status.
Because the primary motivation for running status is to reduce traffic over low-bandwidth connections (suuch as the original DIN current-loop MIDI connection), it is not used in higher-bandwidth transports such as MIDI-over-OSC or MIDI-over-USB.