amplitude

(noun) measurement of highest point (or lowest point) in a wave when compared to resting position

trough

(noun) the area around the lowest point of a wave

crest

(noun) the top of something; sometimes called a "peak"

wavelength

(noun) the distance between two high points (or low points) in a wave

compression

(noun) the state of being squeezed or pressed together

medium

(noun) a substance (like a solid, liquid, or gas) that the energy of a wave can travel though. In the example above, the medium is a flag.