Definition
An epiphenomenon is a stable, repeatable, observable property of a whole system that does not correspond to any single component or stored value. Hofstadter's canonical example: a time-sharing computer slows down at 35 users — but no line of code stores the number 35. The threshold emerges from the interaction of disk seek times, memory contention, scheduler quanta, and user behavior. Nothing in the system knows about it; the system has it.
Why it matters
How it works
An epiphenomenon arises when many components, each following its local rules, interact in a way that produces a stable global pattern. The pattern is not stored anywhere; it emerges from the dynamics. To predict or control epiphenomena you must model the dynamics, not just inspect the components. To change one, you change the interactions — often by altering multiple parameters simultaneously.