It's an abbreviation for accessibility (because there's 11 letters between 'a' and 'y' in the word.)

Similar to i18n and l10n for internationalisation and localisation.

Interestingly unlike w3c, so it's not standing for aaaaaaaaaaay :)

My point was that it's inaccessible: "accessibility", "internationalization" and "localization" are clear to everyone, but "a11y", "i18n" and "l10n" are not at all obvious even to native English speakers, and especially those using screen readers.

I've never understood why these shortenings are so common. Is it just to avoid the mental load of having to remember how to spell long words? Isn't the mental load of having to remember which number goes with which word worse?

It's like a name drop. Saying "a11y" instead of "accessibility" let's other people know what you're familiar with the industry jargon.

Same reason people use contractions. It's faster and conveys equivalent meaning.

A11y is probably the easiest one to remember because it looks like ally. Which is what you're being by worrying about accessibility when you yourself don't rely on the standards.

You only have to remember two digits instead of the correct order of 10–18 letters. It’s probably also a lazy typist thing: four characters instead of 20.

My bad, I had read the parent quickly & thought they'd said "I still don't know what it means" rather than "I still know what it means" so just wanted to expand the jargon for the thread here.

Agree with you re: screenreaders.