A mode in which a MIDI message receiver will receive and act on any data that arrives on any channel. Omni mode was often used in the early days of MIDI to compensate for transmitting devices which had limitations on which channels they could transmit or receive on, and some old synths (such as the Juno-106) powered up with omni mode on by default. However, in the era of multitimbral synths using multiple channels for different purposes, the use of omni mode often causes more problems than it solves, and most newer synth designs no longer implement omni mode.
Two MIDI continuous controller messages are defined for enabling and disabling omni mode. Sending a CC message of type 125 is defined to turn on omni mode. Sending a message of type 124 turns it off; note that all synths on the cable that are in omni mode will be turned off regardless of what channel they are on, since by definition they are ignoring the message's channel number. (The data byte is ignored on both of these messages.) Many newer synths do not implement these messages at all.