Traditionally, only a Jewish man can initiate divorce proceedings. Hence an agunah is a woman whose husband has refused her a divorce. She is unable to remarry, though he is permitted to, and any future children she had would be considered mamzerim (a legal category of persons who may not marry except among themselves).
A woman whose husband has not given her the divorce (GET) papers because he was unwilling for his own reasons, or unable to because he disappeared through war or other action. She cannot remarry until she is legally divorced according to Jewish law.
Agunah (plural: agunot, literally 'chained women') is a term in halakha (Jewish religious law) for a woman who wishes to be divorced from her husband, but who is unable to obtain an official bill of divorce under religious law (known as a Get), because her husband is either unable to be located, or unable or refuses to grant her it.