I found a marriage record* where the woman has the title of "Mistress". What does that title mean in this case? I know that title of "Mistress" has multiple meanings.
The woman was previously married (the previous husband died) and was born in France. I suppose one of those facts is relevant, or perhaps both.
The marriage occurred in Montana on Dec 7th, 1923.
* The record on the right.
I caution against trying to over-interpret the term in the light of modern usage.
It may signify nothing more than the fact that she has been married previously. She is Mistress M(?)eisstin because she is no longer Miss McPherson but nor is she still Mrs Meisstin.
One might have expected the same courtesy to be offered to the divorcee on the facing page, but note that the two records were created by different Deputy Clerks. I suggest you step through some more images in the set to see how other "previously married" ladies are described.
I incline to the simplest explanation: it denotes that she was (previously) married. It may be that, as she was French, her accent was confusing or she gave herself a slightly unusual title. I would check to see if other widowed brides in the records had the same title.
ETA: This answer was being posted at the same time as https://genealogy.stackexchange.com/a/3434/104, so vote for Fortiter's answer if you think we're both correct.
Mistress did have many connotations, one of which was living with someone out of wedlock. I am inclined to say that she was given the title mistress as she and her soon to be husband were living together before the wedding. I came to this conclusion as she is from France and widowed... If we look into 1920's history, we will notice that many women were moving to the USA to begin new lives after the war. Based on their ages, she was probably a World War 1 bride. She would not have come to the USA without a male companion, and thus was probably living in his residence. Your next step to verify this would be to look for evidence that the husband was in the military (or that one of his siblings was).
You've received great answers. I think it's also valid to point towards the first definition that you linked to from Merriam-Webster.
1 : a woman who has power, authority, or ownership: as
a : the female head of a household
b : a woman who employs or supervises servants
c : a woman who is in charge of a school or other establishment
d : a woman of the Scottish nobility having a status comparable to that of a master
Meaning that she is an independent woman, possibly of some means or wealth due to the passing of her husband. Perhaps they owned property together that got passed to her name? Try checking land records to see if she is listed as the owner of a title (or business).
Previous marriage and French usage answers it. I do want to disagree with Shayna.... "She would not have come to the USA without a male companion". This is far from true.
Mistresses and marriage: or, a short history of the Mrs gives a well researched insight into the use of the terms Mrs, Miss and the changes over time.
