$\text{ In the reaction, }2\mathrm{Na}+{\mathrm{Cl}}_2⟶2{\mathrm{Na}}^{+}{\mathrm{Cl}}^{-}\text{the half-reaction }2\mathrm{Na}\left(\mathrm{s}\right)⟶2{\mathrm{Na}}^{+}\left(\mathrm{g}\right)+2{\mathrm{e}}^{-}\text{is called }$

Half-reaction that involves loss of electrons is called oxidation half-reaction. 

eg:  $2\mathrm{Na}⟶2{\mathrm{Na}}^{+}+2{\mathrm{e}}^{-}$