I don’t think it’s possible (which is a shame), defining variables seems to only be at the dashboard level.
When I wanted to define a global constant I ended up having to define the same variable on every dashboard. Which is do-able if you only have a handful of dashboards. If you want to change the value in future of course you have to remember to do it to all dashboards that use the variable.
The other work-around depending on your data source would be to define the value in a database and have a variable defined on each dashboard that gets the value from the database. At least then if you ever want to change the value in future you can just change it once (in the database).
I don’t know if this has been requested as a feature or discussed by devs but I think the ability to define variables at the global level would be a good feature.