A plan participant incurs a break in service and is unvested at the time of the break (only had 3 years of service - 5 year cliff vesting - 15 year break).

The participant is rehired by the same employer (covered by same plan). The plan has a 1 year of service requirement for beginning to participate again.

Can the plan require the employee to again complete the eligibilty period he previously completed (even though service was lost due to a break)?

Also, can anyone give me a good source to look to for the definition of a "participant" and a "former participant"?

Thanks for any help.

Most likely this is already addressed in the plan document. My read of IRC 410 is the prior service can be ignored for purposes of "eligibility service", if the plan includes that provision. See IRC 410(a)(5)(D).

Usually, the best definition of participant is in the plan document, but you might also look at ERISA section 3(7).