Can I use win10 license to activate win8.1?

I got me a laptop which has Windows10 on it. For personal reasons, and I do not want to start an argument as to why I would want to "downgrade". Can I activate Windows 8.1 with my Windows10 license?

Short answer - No

Basically every licence is tied to a particular OS and type, Also OEM licences which are the type supplied with new hardware also have additional limitations. You would also have to source drivers for 8.1, which may or may not be available. There have sometimes been included downgrade licences available on previous OS versions( mostly to business, but sometimes to home users), these are not, as far I I know, being offered at all with Windows 10

Thank you for the insight.

Would it be possible for me to install a 8.0 or 8.1 or even 7.0 ISO image from msdn using a temporary license,
upgrade to 10, using my my valid license, then revert back to 7/8/8.1 which would then be valid? - I really do not know how this all works.

Answer in short is that you will have to buy a new license in order to go down to 7/8/ or 8.1.