It's the number of letters between the first and last letter of the word. See also: i18n, l10n.
Origin:
"A DEC employee named Jan Scherpenhuizen was given an email account of S12n by a system administrator, since his name was too long to be an account name. This approach to abbreviating long names was intended to be humorous and became generalized at DEC. The convention was applied to "internationalization" at DEC which was using the numeronym by 1985."
It's similar to the naming convention used by other long, hard-to-spell words for projects. First letter of word, number of letters in the middle, last letter of word.
Interesting. I really dislike that though as I prefer to just learn how to spell the word instead and probably even more mental acrobatics involved. I can see the usage for a product codename to differentiate electrolysis from codename:e10s.
