An intentional facial expression perceived (incorrectly) by the person making it to convey an irresistible lure to their imagined hotness. They believe that by contorting their face into The Smolder, their target will instantly fall victim to their glorious, glorious face. They themselves are unaware of the complete and utter failure that is The Smolder, and live in a persistent state of denial.
"When I rejected Shika's invitation to the dance, she attempted The Smolder in hopes of changing my mind. I wanted to wash my hands."
Another good example is in Disney's Tangled, where Flynn/Eugene attempts The Smolder on Rapunzel in hopes to win her over and escape.