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.
Some Example Z Codes
Code
Meaning
ZAL
Alter your wavelength
ZAP
Acknowledge, please
ZBK
Are you receiving my traffic clear?
ZBK1
I am receiving your traffic clear
ZBK2
I am receiving your traffic garbled
ZBM2
Place a competent operator on this circuit
ZSF
Send faster
ZBW #
Change to backup frequency #
In this example, the ZBK/1/2 is a Z code from the NATO military set, while the others are old C&W Z-codes. A lot of the old C&W codes are derived from mnemonics (ZAL = (a)lter wave(l)ength, ZAP = (a)ck (p)lease, ZSF = (s)end (f)aster, etc),