Defining custom global constants

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Is it possible to define a custom constant like pi? One that is handled symbolically, one that can be used without prior definition...

For example, could it be possible to define "tau" to equal 2*pi for my Sage installation so that when I open my notebook, I can call "tau" without needing to define it in every cell?

And to take it a step further, would it be possible to replace constants? Like pi with tau/2? This isn't as important, but it would be very nice!