True peak
The peak a signal actually reaches between samples, not just the one written in the file.
A sample-peak meter only reads stored values. When a D/A converter reconstructs the continuous waveform, though, it can overshoot between two samples (an inter-sample peak), so a file that never touches 0 dBFS can still clip in a cheap converter or an MP3 encoder. True-peak metering oversamples the signal at least four times and estimates the real peak in dBTP.
The method lives in ITU-R BS.1770, the same standard that defines LUFS, and EBU R 128 caps broadcast at −1 dBTP. Streaming services recommend similar ceilings, which is why every modern limiter has a true-peak switch.