A simplified stand-in used to reason about something too large to hold. Every model is wrong somewhere, and useful anyway.
Old French modelle, from Italian modello, from Latin modulus (a small measure), the diminutive of modus (measure); a simplified stand-in used to reason about something too large to hold whole.
A deliberately incomplete stand-in for something too large to hold whole — wrong somewhere by design, and useful precisely because it was simplified on purpose.