  In the transitional mass range ($\sim$ 8-10 solar masses) between white dwarf
formation and iron core-collapse supernovae, stars are expected to produce an
electron-capture supernova. Theoretically, these progenitors are thought to be
super-asymptotic giant branch stars with a degenerate O+Ne+Mg core, and
electron capture onto Ne and Mg nuclei should initiate core collapse. However,
no supernovae have unequivocally been identified from an electron-capture
origin, partly because of uncertainty in theoretical predictions. Here we
present six indicators of electron-capture supernovae and show that supernova
2018zd is the only known supernova having strong evidence for or consistent
with all six: progenitor identification, circumstellar material, chemical
composition, explosion energy, light curve, and nucleosynthesis. For supernova
2018zd, we infer a super-asymptotic giant branch progenitor based on the faint
candidate in the pre-explosion images and the chemically-enriched circumstellar
material revealed by the early ultraviolet colours and flash spectroscopy. The
light-curve morphology and nebular emission lines can be explained with the low
explosion energy and neutron-rich nucleosynthesis produced in an
electron-capture supernova. This identification provides insights into the
complex stellar evolution, supernova physics, cosmic nucleosynthesis, and
remnant populations in the transitional mass range.
