Transverse wave
A transverse wave is one in which the disturbance is perpendicular to the direction the wave travels. Waves on a string, surface ripples and light are transverse, and only transverse waves can be polarized.
AP Physics 2 EK 14.1.A.4: in a transverse wave, the direction of the disturbance is perpendicular to the direction of propagation of the wave. A pulse sent down a rope moves along the rope while each bit of rope only moves across it. Nothing travels the length of the rope except energy, since EK 14.1.A.1 says a wave transfers energy between two locations without transferring matter.
Polarization is the test that separates the two wave types. EK 14.3.A.2.i says transverse waves can be polarized and oscillate in a single plane; EK 14.3.A.2.ii says longitudinal waves cannot. A transverse wave has a whole plane of directions perpendicular to its travel to pick from, so restricting that choice means something. A longitudinal wave oscillates along its direction of travel, where there is nothing left to restrict.
Light is the case the exam leans on. EK 14.4.A.1.i says electromagnetic waves are transverse because the oscillations of the electric and magnetic fields are perpendicular to the direction of propagation. That light can be polarized is the evidence.
Crests and troughs are visible on a transverse wave, which is why wavelength and amplitude are almost always taught on one.