An inter-sample peak is a peak in the reconstructed analog waveform that sits between two stored digital samples, so it never appears on an ordinary sample-peak meter. When the DAC's reconstruction filter draws the smooth curve through the sample points, that curve can overshoot the highest sample value, sometimes by a decibel or more. A file that looks safe at -0.1 dBFS in the sample domain can therefore clip the converter, or push a lossy encoder into distortion, on playback. True-peak (dBTP) meters catch these by oversampling before measuring. Inter-sample peaks are why mastering and delivery specs set ceilings below 0 — typically -1 dBTP — to leave room for the overshoot.
