invisible

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Impossible to see; not visible.
• adjective Not accessible to view; hidden.
• adjective Not easily noticed or detected; inconspicuous.
• adjective Not published in financial statements.
• noun One that is invisible.

from The Century Dictionary.

• Not visible; incapable of being seen; imperceptible by the sight.
• Out of sight; concealed or withdrawn from view: as, he keeps himself invisible.
• noun A Rosicrucian: so called because of the secret character of the organization.
• noun One who rejects or denies the visible character or external organization of the church; specifically [capitalized], a name given to certain German Protestants because they maintained that the church of Christ might be, and sometimes had been, invisible.

from the GNU version of the Collaborative International Dictionary of English.

• noun An invisible person or thing; specifically, God, the Supreme Being.
• noun obsolete A Rosicrucian; -- so called because avoiding declaration of his craft.
• noun (Eccl. Hist.) One of those (as in the 16th century) who denied the visibility of the church.
• adjective Incapable of being seen; not perceptible by vision; not visible.
• adjective Not visible due to an inherent property, such as lack of color;
• adjective hidden from view; out of sight
• adjective Not perceptible due to lack of light
• adjective Too small or too distant to be perceived.
• adjective Hidden from the public.
• adjective imperceptible to the mind.
• adjective (Zoöl.) a small, shy singing bird (Myadestes sibilons), of St. Vincent Islands.
• adjective a very dark shade of green, approaching to black, and liable to be mistaken for it.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective physics, optics Unable to be seen; not visible.
• adjective Internet Apparently, but not actually, offline.
• adjective psychology That is ignored by a person.
• noun obsolete An invisible person or thing; specifically, God, the Supreme Being.
• noun obsolete A Rosicrucian; so called because avoiding declaration of his craft.
• noun obsolete One of those (as in the 16th century) who denied the visibility of the church.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective not prominent or readily noticeable
• adjective impossible or nearly impossible to see; imperceptible by the eye