As in "basking in the afterglow" - the warm, glowing feeling of bliss that lingers following orgasm after engaging in happy sex.
It should be clear that the affection hasn't really finished, but the physical activity is merely winding down. The persons involved should still emotionally focused on each other, doing things like cuddling, spooning, holding hands, making eye contact, and similar indications of affection, possibly also (literally) sleeping together through the night. If the partners have disengaged from one another, then the "encounter" really is over, and this tag should not be used.
Contrast with after sex, which is for graphic, physical evidence of prior sex. Afterglow should be based on the apparent emotional state of those involved. Also note that, while they can coexist, these two tags tend to be mutually exclusive due to reasons of conflicting mood and style.
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