What is the ETH killer?

Could you please elaborate on the term 'ETH killer'? What exactly does it refer to in the realm of cryptocurrency and finance? Is it a specific project or a general category of competitors that aim to overtake Ethereum in terms of popularity or functionality? How does the community typically define such a contender, and what characteristics does it possess that might threaten Ethereum's dominant position in the market? Furthermore, could you discuss some of the potential candidates that are currently being touted as potential 'ETH killers' and why they are considered so?