Firstly, it’s a french term defining a lady who is not married. Now we use it just for a lady.
I met a lot of beautiful mademoiselles when I was in Paris.
mademoiselle: define #2
A French diminuitive of “Madame”; “Mademoiselle” means “My little lady”. It once was used to signify an unmarried woman, but concidering that it is a diminutive of “Madame”, now it is just used for girls and young women, and the professional title for women is “Madame”.
Still, Mademoiselle is used for young women by their admirers.
“Mon Dieu! Bonjour, Mademoiselle, je m’appel…”