Z Code (like Q Code and X Code) is a set of codes used in CW, TTY and RTTY radio communication. Actually, there are different sets of Z-codes: one originally developed by Cable & Wireless Ltd., for commercial communications in the early days of wire and radio communications, another one independently developed by NATO forces later for military needs and use. The NATO Z Code is still in use today, and is published in the unclassified document ACP-131. There are other set of codes internally used by USSR’s military and other operating agencies. The old C&W Z Codes are not widely used today.
List of Z codes http://www.kloth.net/radio/zcodes.php