What is the X in X-mas?
a late-developing letter in English. Called ipsilon in German, upsilon in Greek, the English name is of obscure origin. The sound at the beginning of yard, yes, yield, etc. is from Old English words with initial g- as in got and y- as in yet, which were considered the same sound and often transcribed as a character that looks something like 3 (but with a flat top and lower on the line of text), known as yogh. The system was altered by French scribes, who brought over the continental use of -g- and from the early 1200s used -y- and sometimes -gh- to replace 3. There's a good, in-depth discussion of yogh here. As short for YMCA, YWCA, YMHA first recorded 1915.
The symbol for the element yttrium.
The symbol for yttrium.