What kinds of ions do a sodium atom and a chlorine atom become when a valence electron is transferred from one to the other?
The chlorine atom is reduced; the sodium atom is oxidized.
Heat sodium metal up in an atmosphere of chlorine gas, and a redox reaction occurs; a transfer of electrons.
Sodium (gas) is oxidized.
Chlorine gas is reduced.
Add these reactions together, and we get:
This is certainly a redox reaction, and the individual reaction enthalpies may be precisely measured and defined.