ad. In a way that can not be seen; in an invisible manner.
the GNU version of the Collaborative International Dictionary of English
ad. In an invisible manner.
The Century Dictionary and Cyclopedia
In a manner to escape the sight; so as not to be seen.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
ad. without being seen
Word Usage
"New research by University of Minnesota psychologists shows how social support benefits are maximized when provided "invisibly" -- that is, without the support recipient being aware that they are receiving it."