환승이 · 환승을
transfer (between buses or trains)
Example
삼십 분 안에 환승을 하면 요금이 무료예요.
If you transfer within thirty minutes, it's free.
Word parts
환 change+승 ride
Literally 'change rides'.
Good to know
In speech, 갈아타다 is more common; 환승 is what you hear in announcements and see on signs.