undefended vs open

undefended is not a noun while open is a noun.

Word	Noun	Adjective	Verb	Adverb
undefended	No	Yes	No	No
open	Yes	Yes	Yes	No

As adjectives, undefended and open are synonyms defined as:

Other synonyms of undefended include assailable, undefendable.

undefended (noun)	open (noun)
	information that has become public
	a tournament in which both professionals and amateurs may play
	a clear or unobstructed space or expanse of land or water
	where the air is unconfined

undefended (adjective)	open (adjective)
not defended or capable of being defended	ready or willing to receive favorably
	affording unobstructed entrance and exit; not shut or closed
	affording free passage or access
	(set theory) of an interval that contains neither of its endpoints
	open and observable; not secret or hidden
	used of mouth or eyes
	ready for business
	not having been filled
	(of textures) full of small openings or gaps
	openly straightforward and direct without reserve or secretiveness
	not requiring union membership
	open to or in view of all
	possibly accepting or permitting
	without undue constriction as from e.g. tenseness or inhibition
	having no protecting cover or enclosure
	affording free passage or view
	with no protection or shield
	accessible to all
	not sealed or having been unsealed
	not brought to a conclusion; subject to further thought
	not defended or capable of being defended

undefended (verb)	open (verb)
	make available
	become available
	make the opening move
	cause to open or to become open
	become open
	spread out or open from a closed or folded state
	display the contents of a file or start an application as on a computer
	afford access to
	start to operate or function or cause to start operating or functioning
	begin or set in action, of meetings, speeches, recitals, etc.
	have an opening or passage or outlet