The district dd12 was in last year used a single grade skip as the definition of radical acceleration which is nuts in my books. I made them change that in her ALP so it just read "accelerated into 6th grade" as opposed to "radically accelerated into 6th grade." It does seem like there is some subjectivity in defining "radical acceleration," though. I'd go with at least 2 yrs and maybe 3+ like Dottie mentions.