A character is notably desirous of the exclusive affections that another receives from a potential lover.

Notes:
In cases of infidelity, for example, it must be clear that resentment is over emotions and not some other concern. For example, a spouse who no longer desires their partner may still be upset about infidelity for the social stigma, or for practical reasons. They are not, then, romantically jealous.

Examples:
In blackmirror1x03 "The Entire History of You", Liam was exceedingly jealous of Jonas over the way Jonas' interest in his wife Fion clearly had solicited a positive response in the past, and perhaps still did. He might not even be the father of their daughter Jodie.

References:
https://en.wikipedia.org/wiki/Jealousy#Romantic_jealousy