  A search for inelastic boosted dark matter (iBDM) using the COSINE-100
detector with 59.5 days of data is presented. This relativistic dark matter is
theorized to interact with the target material through inelastic scattering
with electrons, creating a heavier state that subsequently produces standard
model particles, such as an electron-positron pair. In this study, we search
for this electron-positron pair in coincidence with the initially scattered
electron as a signature for an iBDM interaction. No excess over the predicted
background event rate is observed. Therefore, we present limits on iBDM
interactions under various hypotheses, one of which allows us to explore an
area of the experimental search for iBDM using a terrestrial detector.
