noun
regular service; scheduled service
Refers to transportation or delivery services that operate on a fixed schedule, such as flights, buses, ferries, or mail delivery.
この港には週に二回、定期便が入る。
A regular service calls at this port twice a week.
離島への定期便は一日一便
There is only one scheduled service per day to the remote island.
便 means 'service' or 'flight/shipment' and is often used in compounds like 航空便 (airmail) or 船便 (sea mail). 定期便 combines this with 'regular' to mean a scheduled service.
Compound of 定期 (ていき, 'regular, fixed period') and 便 (びん, 'service, mail, flight'). The term is straightforward and modern, with no obscure historical derivation.