(Latin: in, not; regula, rule; not according to rule)
An impediment introduced by ecclesiastical law preventing one from entering the clerical state, or from the exercise of any orders already received.
The Church intends irregularities primarily as a safeguard to the dignity of her ministry, though sometimes they may appear to share the juridical nature of a penalty.
Irregularity may rise either ex defectu (from defect) in the subject, as illegitimacy, bigamy, bodily defect; or ex delicto (from crime), as apostasy, heresy, homicide, attempted suicide.
The new Code of Canon Law limits once for all the number and kind of irregularities as pertaining to the common law of the Church.
New Catholic Dictionary