Electronic Conspicuity (EC) is an umbrella term for a range of technologies that, in their most basic form, transmit the position of the host aircraft to other airspace users operating compatible equipment. More advanced devices can also transmit and receive, displaying and alerting pilots to other/conflicting traffic who have compatible EC devices. EC devices turn the traditional ‘see and avoid’ concept into ‘see, BE SEEN, and avoid’.  You can learn more about the program on the UK CAA’s Electronic Conspicuity page.
EC devices are intended for voluntary carriage on registered and non-registered UK Annex II  aircraft, non-complex EASA aircraft of <5700kg MTOM and for gliders and balloons (including those covered under ELA 1 and ELA 2) within uncontrolled UK airspace.