When one, or preferably many, muggles dressed as Lord Voldemort simultaneously reign terror at an event and sufficiently disturb the peace of said event. Any form of Voldemort is fair game. Voldemorts are often found screaming unforgiveable curses (especially crucio) at women, children, and mudbloods.
Six of my buddies and I were voldemorting the shit out of my cousin's wedding last weekend. Poor bride's maids didn't stand a chance against my cruciatus curse, and I think Tom Riddle may have killed that adorable ring bearer.
Voldemorting is when you're fucking a chick doggy style and right when you're about to cum you pull out and make an evil face on the back of her head with your semen.
"I was having sex with a Hermione cosplayer and gave her a good ol' fashioned Voldemorting. She didn't appreciate it."