The loss of electrons.
Oxidation is defined as the loss of electrons. A simple oxidation reaction might happen during electrolysis, and at the anode.
For example, chloride ions get oxidized into chlorine gas with the following half-equation:
In terms of electrons, Oxidisation is the LOSS of electrons.
The easiest way to remember this is by the acronym
This stands or oxidisation is lost, reduction is gain.
oxidisation is usually seen at the anode during electrolysis as negatively charged ions are attracted towards it where they lose electrons to become atoms.
Reduction and oxidisation are also seen in redox / displacement reactions where oxidisation and reduction will happen simultaneously.