40 ILCS 5/20-109
"Pension credit": Credit or equities acquired by an employee in the form of contributions, earnings or service as defined under the law governing each of the systems in which he has credits or equities, except credits and equities (1) of less than one year in any one system, except that this one-year limitation shall not apply to (A) employees who transfer or are transferred, as a class, from one participating system to another or who are persons to whom Section 14-108.2a or 14-108.2b applies or (B) persons who move from participation with a school district as a teacher aide under Article 7 to participation under Article 16; or (2) which have previously been forfeited by acceptance of a refund or which have been applied towards a retirement annuity and have not been reestablished in accordance with the law governing the system from which the refund or retirement annuity had been received. If a retirement system provides no refund of contributions, the pension credit in the case of any employee who has participated in that system shall be considered effective for the purposes of this Article.
40 ILCS 5/20-109