The twenty-sixth letter of the alphabet.
In ASL culture, we never sign "and" between "y" and "Z". We just fingerspell from A to Z without doing 'and' between the last two alphabetical letters.
Interesting, this bilingual five-year-old child in the video has been using 'and' between Y and Z since she was 3. She was influenced by English-speaking culture as she learned ABCs in preschool, whereas I rarely taught her ABCs in alphabetical order, just random alphabetical letters.