Audiophile slang
Burn-in
Also called break-in, run-in
Burn-in is the belief that gear sounds better after hours of use as its parts settle. Measurements show a small change in speaker suspensions and little or none in headphones and electronics; many owners still hear a difference, which sceptics put down to adaptation.
The mechanical part is real but small. A loudspeaker’s suspension loosens slightly in its first hours, shifting its resonance by a few hertz, and large dynamic drivers can show a measurable change. Those changes are usually below what a listener would notice from one day to the next.
Measurements of headphones before and after hundreds of hours generally show no meaningful difference, and controlled comparisons of a "burned-in" unit against a fresh one have not found listeners able to tell them apart. What does change is the listener: ears adapt to a new tonal balance within days, which feels exactly like the headphone improving.
Burn-in for cables, DACs and amplifiers has no measured basis. Some makers recommend it; measurements have not shown a mechanism in electronics or cables. The honest summary is that a product you dislike on day one will probably still have the same Frequency response on day thirty, and you may like it anyway.