na-adjective, noun which may take the genitive case particle 'no', noun
irregular; indeterminate; staggered
Describes schedules, services, payments, or time periods that are not fixed or regular. Often used with nouns like スケジュール, 休み, 支払い, or 間隔.
This bus runs on an irregular schedule.
He does irregular work.
This bus runs on an irregular schedule.
Payments are made irregularly, in a lump sum.
The salary payments became irregular.
He takes days off on an irregular basis.
臨時 describes something arranged specially for a one-off occasion (extra, ad hoc), whereas 不定期 simply means the recurring schedule has no fixed interval.
定期的 means happening at fixed, regular intervals, the direct opposite of 不定期, which means the timing is not fixed.
定期 describes something happening at a fixed, regular interval, while 不定期 means it happens irregularly or with no set schedule.
不定期 means "irregular, not on a fixed schedule," the opposite of 月例's sense of a fixed monthly recurrence.
Compound of 不 (fu, 'not') + 定期 (teiki, 'fixed period'). The reading ふていき follows standard on'yomi.