The mel unit of pitch. A pure tone at 1000 Hz and at an intensity of 40 dB above the listener’s threshold is defined as having a pitch of 1000 mels. A frequency judged to be one tenth of the pitch of a 1000 Hz pure tone has a pitch of 100 mels and so on. A commonly used formula relating pitch in mels to frequency in Hz is:

The graph below shows the relationship:

Image credit: Krishna Vedala. Used under this licence.