Specify a list of tags to keep hidden: These tags will be invisible to users, such as in tag clouds or in post tags lists. This is useful when using tags to control behavior of your blog, or when you wish to maintain groupings of posts out of the public eye.
This does not prevent tag archives for hidden tags from being accessible, only hides tag links from tag lists and tag clouds.
Capable users (with the see_hidden_tags capability,) can see hidden tags in tag lists (but NOT tag clouds.) Hidden tags will receive the .hidden-tag class and are grayed out by default.