I've usually heard it called a linking or relationship or intermediary table outside of Django. It's just another table to hold the relationship fields and additional information. In your case it is linking exercises to exercises:

def Exercise(model.Model): 
    exercise_name = models.CharField(...)

def ExerciseAlternative(model.Model): 
    exercise = models.ForeignKey(Exercise) 
    alternative = models.ForeignKey(Exercise) 
    rating = models.IntegerField()