The degree to which successively-printed colors (or images) are accurately positioned with respect to each other. Accurate register ensures that a final printed piece has the effect of a "single image," with no color gaps or overlaps. Register is initially set by correct exposure, positioning, and mounting of printing plates (or other image carriers), but proper register on the printed piece can be mitigated against by many different variables, such as dimensional changes in the substrate due to changes in moisture content and/or mechanical stretching, deficiencies in the press feeding section, web tension, or changes in the image-carrying portions of the press, such as blanket compression (in offset lithography) or plate swelling (in flexography), among many other factors.
Depending on the application, accuracy of registration may be more or less necessary. See Absolute Register, Close Register, Commercial Register, Hairline Register, Lap Register, and Loose Register. Inaccuracies in registration are known as misregister. Process color printing is usually performed utilizing register marks, small shapes on successive plates which aid in the setting of proper register.
'Register is also used to refer to a storage area in a computer. See Data Register.
[In C, variables may be declared as register, instructing the compiler to keep the value of these variables in the register of the CPU rather than in memory, where variables are conventionally stored. Operations performed on register variables are therefore very fast.]