Objects can be in one of three visibility states as indicated in the Objects palette:
Visible - object is visible and can be selected.
Ghosted - object shown in light grey wireframe and can’t be selected.
Invisible - object is not visible and can’t be selected.
You can manually change the visibility state by using the Ghost and UnGhost tools.
Another method is to click in the 2nd column of the Objects palette.
Objects can automatically change to ghosted with certain operations such as
Booleans. For example, if you Boolean Difference object “Cube” with object
“Cylinder” the result is a new object “Cube” and the original two operands are
ghosted and remain in the Objects palette. If you need to access the original
objects again simply UnGhost them.
To temporarily hide all ghosted objects in your scene turn ON the Hide Ghosted
option in Display Options palette
To permanently delete all ghosted objects from your scene select Clear All Ghosted
from the Edit pull-down menu This deletes objects ghosted manually with the Ghost
tool and objects ghosted as a result of an operation.
To delete only ghosted objects created as a result of an operation select Clear
Ghosted Tool Operands from the Edit pull-down menu. All objects ghosted manually
will remain in the Objects palette.