X Bitmap

A black and white bitmap graphics format used by UNIX. Its
filename extension is .XBM

X Protocol

The protocol used by the X Window System for exchanging
messages between clients and servers

X Remote

A serial line protocol for the X Window System


Xwindows

A GUI for Unix, very much like a windows interface.
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YAHOO

An online World Wide Web directory, found at http://www.yahoo.com 

Yahoo Pager

Yahoo!'s instant messaging system

Yahooligans!

A kid safe version of Yahoo's directory. 

Y Modem Protocol

Similar to X-modem (see X Modem Protocol), but lets you send a 
batch of files with a single command. Also uses an error checking
process called CRC (see CRC).