It needs to be in the environment of the user executing the scripts that need it.
If you want it just for yourself, you could edit your .profile to add...
MERAKI_API_KEY="whatever it is"
export MERAKI_API_KEY
If you will be running the Python scripts from inside a shell script wrapper (for instance if you were using cron to run the script periodically) you can put the same lines in the shell script.
At the linux shell prompt, you can type 'env' to see your environment.