The romantic interest (also called love interest) is a stock character, an object of romantic admiration and attraction for the main character(s), or hero/heroine e/s. It is also the plot element, the romantic subplot, thus introduced.
This person is often a female acquaintance of the protagonist, as in the case of Jane in Tarzan but can be a male, as in the case of Disney's Pocahontas. The primary characters and romantic interests may be the two main characters, as in the case of Mr. & Mrs. Smith.
This term is often used in movie reviews, less so in classic literature, though such characters are common. Nearly all of the Disney animated features have main characters, villains and romantic interests that even children can easily identify.
"Love interest" is a more distant term than "The Hero's Beloved" or "Sweetheart", often used earlier for such characters. (The replacement of "hero" with "protagonist" or "main character" serves a like purpose").
The term "love interest" is also sometimes used in real life to designate someone's potential sweetheart or target of infatuation.