I believe in this context it refers to Enhanced Control Interface (it's not a particular device), a way to discern/control what is attached to the A/V jack.
RX51_JACK_HP, /* headphone: stereo output, no mic */ RX51_JACK_HS, /* headset: stereo output with mic */ RX51_JACK_MIC, /* mic input only */ + RX51_JACK_ECI, /* ECI headset */ RX51_JACK_TVOUT, /* stereo output with tv-out */ };
static const char *jack_function[] = {"Off", "Headphone", "Headset", - "Mic", "TV-OUT"}; + "Mic", "ECI Headset", "TV-OUT"};