i Register
In some senses, digit is marked as obsolete, archaic, historical. Watch for register when choosing this word.
ADJ
binary, decimal
My password contains eight random digits, including both letters and numbers.
QUANT
sequence, series, set, string
The PIN code requires you to enter a string of six digits.
PHRASES
three, four, etc. digits long
My PIN code is four digits long, so it's easy to remember.