  Frequency is a central concept in Mathematics, Physics, and Signal
Processing. It is the main tool for describing the oscillatory behavior of
signals, which is usually argued to be the manifestation of some of their key
features, depending on their nature. For instance, this is the case of
Electroencephalographic signals. Hence, frequency is substantially present in
the most common methodologies for analyzing signals, as the Fourier Analysis or
the Time-Frequency Analysis. However, in spite of its importance as a keystone
in Signal Processing, and its seemingly simple meaning, its mathematical
foundation is not as straightforward as it may seem at first glance. A naive
interpretation of the different mathematical concepts modelling frequency can
be misleading, as their actual meanings essentially differ from the intuitive
notion which are supposed to represent. In our opinion, this circumstance
should be taken into account in order to develop appropriate signal analyzing
and processing tools in some applications. In the current text we discuss this
topic, with the main goal to draw the attention of the mathematical and
engineering community to this point,often overlooked.
