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Abiogenesis: The process by which dead matter formed life. There are a plethora of theories as to how.

LUCA: The Last Universal Common Ancestor of all currently living beings. We can infer some characteristics it may have possesed. Note that LUCA is not the same as the first living being, but instead the last life form from which we all (plants, animals, fungi, prokaryotes, archea) descend.

RNA world: The theory of the origin of life explained by the other commenter.