I guess if anyone is up to the task of defining what "is" is it's Bill Clinton. But I still think even the 22 Amendment is pretty clear.
Amendment XXII
Section 1. No person shall be elected to the office of the President more than twice, and no person who has held the office of President, or acted as President, for more than two years of a term to which some other person was elected President shall be elected to the office of the President more than once. But this article shall not apply to any person holding the office of President when this article was proposed by the Congress, and shall not prevent any person who may be holding the office of President, or acting as President, during the term within which this article becomes operative from holding the office of President or acting as President during the remainder of such term.
Contrast that with Article II's statement of eligibility: "No person except a natural born citizen, or a citizen of the United States, at the time of the adoption of this Constitution, shall be eligible to the office of President; neither shall any person be eligible to that office who shall not have attained to the age of thirty five years, and been fourteen Years a resident within the United States." (My italics)
As I've read the two amendments, what they mean to me is that Clinton could become President if he were Vice-President, but then he could not run for re-election.