The STFT is a mapping from a
one-dimensional function (the domain, which is a function of
time) to a two-dimensional
function (the range, which is a function of time and frequency).
Now suppose, rather than holding 
constant (4),
we also allow it to be a parameter.
The new mapping we so obtain is a mapping from the one-dimensional
domain (time) to a three-dimensional range
(time, frequency, and log-scale) that we previously referred to as the
TFS parameter
space
(Fig. 1(a)).