Refers to a business that arranges travel, sells tickets, and provides related services. The term is straightforward and commonly used.
I booked a flight ticket at a travel agency.
旅行会社 is the general word for a travel company or agency, while 旅行代理店 more precisely names the physical office or agency that arranges bookings.
Compound of 旅行 (travel) + 代理店 (agency). The word is transparent and modern, with no obscure historical derivation.