What are Ethereum killers?

In the dynamic and ever-evolving world of cryptocurrency, the term 'Ethereum killers' has gained significant attention. Could you elaborate on what this phrase actually refers to? Are these potential competitors that aim to overtake Ethereum in terms of technology, adoption, or both? Do these projects pose a legitimate threat to Ethereum's dominance in the smart contract and decentralized finance space? What are some of the key features or innovations that these 'killers' bring to the table, and how do they compare to Ethereum's capabilities?