Up till now, I have been using "i18n" and "i10n" interchangeably to mean localization. Yep - I didn't know that they are numeronyms (I should have known that when I heard about k8s), that l10n starts with a l, or that i18n and l10n actually mean different things.
In i18n, 18 stands for the number of letters between the first i and the last n in the word internationalization.
The same rule also applies to l10n.
Other numeronyms include g11n (globalization), a11y (accessibility), and i14y (interoperability).
Localization refers to the adaptation of a product, application or document content to meet the language, cultural and other requirements of a specific target market (a locale).
Internationalization is the design and development of a product, application or document content that enables easy localization for target audiences that vary in culture, region, or language.