from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Between digits (as when dialling a number).
from The Century Dictionary and Cyclopedia
- n. The fold between any two adjacent fingers or toes.
inter- + digit (Wiktionary)
It varies with the equipment used since ESS can handle higher speeds and doesn't need interdigit intervals.
With this in mind, the Bell standard for MF pulses and interdigit intervals is around 75 ms.