Autodesk AUTOCAD 2006 Command Reference Manual page 827

Shortcut menu: Right-click in the command window, or (with no commands
active and no objects selected) right-click in the drawing area, and choose
Options.
Command line: options
Controls whether right-clicking in the drawing area displays a shortcut menu
or has the same effect as pressing ENTER . If you are accustomed to using the
right-click method for ENTER while a command is active, you might prefer
to disable Command shortcut menus from this dialog box. You can also control
shortcut menus by using the SHORTCUTMENU system variable.
Turn on Time-Sensitive Right-Click
Controls right-click behavior. A quick click is the same as pressing ENTER .
A longer click displays a shortcut menu. You can set the duration of the longer
click in milliseconds.
Default Mode
Determines what happens when you right-click in the drawing area when no
objects are selected and no commands are in progress.
ENTER
Disables the Command shortcut menu. As a result, right-clicking in the
drawing area when a command is in pro-gress is the same as pressing ENTER
.
Right-Click Customization Dialog Box | 803