The term "a11y" is a numeronym that stands for accessibility; it is abbreviated to be a11y is because there are eleven letters between the first and the last letter. One of the reasons why it is abbreviated is that it is easy to spell accessibility incorrectly, especially for people with learning disability such as Dyslexia and ADHD. Also, social media platforms such as Twitter limits character counts, so #a11y is what people use for hash tagging.