If I delete music from my MacBook Air, will that also delete it from iCloud?

If you have Apple Music or iTunes Match there should be a right-click option to Remove Download, freeing up local storage while leaving the track in your iCloud Music Library from where it can be streamed. I would recommend backing up your original copies to an external drive before doing this, so that if you later delete the version from iCloud Music Library in error you would have an option to restore from your backup.

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If you use iTunes Match or subscribe to Apple Music, changes you make will reflect on all devices.