Technology and data formats
ASCII
Abbreviation for American Standard Code for Information Interchange
In short and in plain language
ASCII is an old list of letters and signs for computers.
It only has letters without accents, digits and a few signs. Letters like ü are missing.
A program that only knows ASCII replaces such letters. Or it shows them in the wrong way.
Definition
ASCII is an early character set for computers that only covers letters without accents, digits and some special characters. Umlauts or letters such as Ł are missing. Systems that only handle ASCII replace such characters or display them incorrectly.