an abbreviation of "accessibility," especially used in online spaces such as Twitter where character count matters.

"a11y" is a numeronym — the "11" refers to the number of characters between the A and the Y. "a11y" may variously be pronounced like "ally," "alley," "a-one-one-y," "a-eleven-y," or "accessibility." Other similar numeronyms include i18n, which is short for "internationalization."

Further Reading