An intermediate light switch it the centre switch or switches in a lighting circuit. The two end switches are two way light switches. Any switches in between these two switches need to be intermediate.
This allows the light or lights to be controlled from any switch.
How do I know whether my switch is an intermediate light switch?
If you look at the back of the light switch, and intermediate switch will have two L1 and L2 connection points. Two way light switches will have one L1, one L2 and a COM connection point.
As always, if in doubt please consult a qualified electrician and never try to work on live electrics without switching off at the consumer unit first.
Can I use an intermediate switch as a two way light switch?
Yes, you can use an intermediate light switch as a two way. You would need to attach jump wires, and it is not a cost-effective way to wire a lighting circuit, so we would generally advise swapping out the intermediate switch for a two way light switch if you do not require it for three way switching.