With contributions by MaddieFay. - Thanks!
The term traditionally used to describe the wife of a cuckold; that is to say, a woman who has sex with men besides her husband.
In cuckold culture it has come to be used for any woman who has sex with individuals other than her significant other. This can include a woman in any heterosexual, homosexual, or bisexual relationship with or without the presence of a marriage.
In extremely rare cases, it is used to describe a woman who has an intimate, often dominant, relationship with an individual with which she has no other relationship outside of friendship but with whom she shares all other aspects of a cuckold lifestyle. For instance: a woman who lives with or spends a large amount of time with a man (or woman) who provides domestic and moral support as well as companionship, while she reserves all of her sexual energy for others, could be called that man's (or woman's) Cuckoldress.
The term is often used to describe a position of authority, similar to Dominatrix or Mistress.