We now develop a new vocabulary to describe this new kind of periodicity. It will be directly analogous to our description of periodicity in time. The only difference is in terminology and the fact that we use measures of distances instead of times.
: The distance we must travel down the
wave for a full period or cycle. Wavelength is typically measured
in meters, so that the basic unit is 
.
: The number of cycles or waves which
occur in a unit distance. The unit of wavenumber is typically
.
radians of phase
with each cycle. The unit of angular frequency is thus typically
. Because radians carry no dimension, we
frequently omit the radian and write the unit of wave vector as simply
.