Absorption Spectrum Definition:

In IR spectroscopy as well as UV-VIS spectroscopy, a plot that measures the percent transmittance or absorption as a function of frequency.

Absorption Spectrum Explained:

An IR spectrometer is designed to measure the % transmittance or intensity. As a result, the plot formed has the “% transmittance” on the Y axis, while the wavenumber is on the x-axis.

Uses of IR Absorption Spectrum:

The IP absorption spectrum provides important information on functional groups within a compound. Different functional groups have different absorption frequencies. Here is a sample chart:

Source: https://socratic.org/questions/how-does-infrared-spectroscopy-identify-functional-groups