Transverse Vector: Definition & Meaning

[Harel]

Hey, in sean carroll's book about GR he said something about transverse vector, can someone explain me what a transverse vector is? Also he said:"Remember that the vectors are defiend only in the two transverse dimensions" what is the meaning of transverse dimension

 

[BvU]

In relativity, as in waves, transverse is perpendicular to the direction of motion