1. The celebration (on a chosen day) of your length of friendship with a particular friend. Like an anniversary but without the promise of sex afterwards.
2. Acknowledging the tenure of friendship you've had with someone by arranging a special "hang-out" day.
3. A platonic date to honor your friendship.
"Hazel and I are celebrating our freniversary by going out to dinner at CPK then watching a movie."
"Dude did you know we've been friends now for over 6 years?"
"No way! We should have a freniversary to celebrate how awesome we are!"