By current market standards, there are two different definitions of the romance. One is defined by mass-market paperback houses like Harlequin and Leisure, and this definition is what most romance readers expect when they read a book called a romance. For the sake of clarity, I will call this a "standard romance."
The standard romance should include a happily ever-after ending (HEA). At the end of the book, the one man and one woman relationship ends with the promise of monogamy if not marriage. The hero and heroine should be deserving of those titles by their actions and behavior.
The emotional plot turning point, the crucible in which their relationship is tested, is the center of the story. The reader sees that if this one problem is solved before the HEA, there will, indeed, be a happily ever after. Problems can be things such as trust or commitment, and self-worth issues. Before the hero and heroine can have a HEA, they have to face this problem, grow, and change for the better to earn that happily ever after.
These elements must be at the core of the novel to be a romance.
The second definition of romance is the mainstream romance. Another good name for these romances would be tearjerker romances. Most of these books are written by men. Think BRIDGES OF MADISON COUNTY and all books by Nicholas Sparks.
The unhappy ending is as expected here as the HEA is expected in the other kind of romance. The plot rarely follows linear time, and the ending is known before the beginning usually in a frame story.