The NCAA released an excellent Educational Column on the missed term exception. That exception helps athletes stay eligible when they miss a term by prorating the 18-hour rule by 9 hours per term of attendance. Put another way, the athlete gets nine “phantom credits” that are applied to the requirement that athletes must earn 18 semester hours or 27 quarter hours during the regular academic year (fall and spring only).
The Ed Column specifically looks at one of the requirements to use the missed term exception: that at the time the athlete did not enroll, the athlete was academically eligible. If you start to get confused looking through the missed-term exception, one thing to keep in mind is that if an athlete misses an academic requirement that spans multiple terms (like the 18 hour rule), they regain eligibility by dropping the oldest term being evaluated, then seeing if they were eligible after the following semester using the two most recent terms.