y
y-intercept Set x to 0 and solve for y. (0,y)            
Yagi antenna Directional antenna used for long distances.   Wikipedia         
Yahoo!  Started by Jerry Yang and David Filo January, 1994 as Jerry and David's guide to the World Wide Web. Incorporated as Yahoo in March, 1995.  Wikipedia  Website       
Yahoo! User Interface Library (YUI) An open-source (BSD license) JavaScript library.   Wikipedia  Website       
yank y
Copy text inside of vim.   Vim Wiki         
Yorkfield Codename for some Intel Core 2 Quad and Xeon processors using the Penryn microarchitecture. First released March 15, 2008.  Wikipedia         
Yosemite (OS X 10.10) Released October 16, 2014.  Wikipedia         
yottabyte (YB) 1000 zettabytes   Wikipedia