A love interest is the person with whom a character is actually in an acknowledged canonical, romantic relationship. Even if it's unrequited. A character's desire for their love interest is often an important plot device.
Someone that a character is made to pine after (or have a relationship with) in a fic is not a love interest, but a lust object. It is interesting to note that agents are noted as having lust objects rather than love interests (with the exception of relationships between agents and other agents or non-canon characters).