means A living thing—any single life form, from a bacterium to a blue whale—that functions as a self-sustaining, organized whole.

from From the same root as 'organ' and 'organize,' tracing back to Greek 'organon,' meaning a tool or instrument (and a cousin of 'ergon,' work). The idea is a body made of working parts, each an instrument doing its job. 'Organism' itself is a relatively modern formation—an 18th-century coinage built on that ancient sense of a coordinated set of working tools.