Originally  the term oxidation was used to describe the addition of Oxygen to an element or a compound. Many elements present in the atomosphere. This is the  reason why they commonly occur in the earth crust as their oxides. Two oxidation processes according to the above definition of oxidation are
$$\begin{gathered} 2\mathrm{Mg}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{MgO} \left(\mathrm{s}\right) \left(1\right) \\ \mathrm{S}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{SO}}_2\left(\mathrm{g}\right) \left(2\right) \end{gathered}$$
In reaction (1) and (2) the elements Magnesium and Sulphur are oxidised on account of addition of oxygen to them. Similarly, methane is oxidised owing to the addition of oxygen to it
${\mathrm{CH}}_4\left(\mathrm{g}\right)+2{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{CO}}_2\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{I}\right) \left(3\right)$
In equation (3) hydrogen has been replaced by oxygen. This has prompted chemists to reinterpret oxidation in terms of removal of hydrogen from it and therefore the scope of term Oxidation was broadened to include the removal of hydrogen from a substance.