In biology, a cell, tissue, or organ that responds to a stimulus and carries out an action, like muscles responding to nerve signals.
From 'effect' plus '-or' (agent). Modern biological terminology standardized on this spelling variant, though 'effecter' remains an older synonym.
Your muscles are 'effectors'—they're the endpoint of your nervous system where electrical signals finally become physical action. The word choice is brilliant because it emphasizes that biology is about causation and change, not just static structures.
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