B Definition of unshifted spectral lines?

[Pilutiful]

Hi,
N00b question :-)
I have read articles about redshift and blueshift and have a good idea about them. What I would like to know is how you decide if something is "unshifted" - what is the definition of this? is there a standardized way of defining/mapping an element's unshifted spectral lines?
Hope this makes sense...

 

[Bandersnatch]

You can measure what the absorption lines of e.g. hydrogen are in laboratory conditions. This is your standard of reference.

 

[Pilutiful]

You can measure what the absorption lines of e.g. hydrogen are in laboratory conditions. This is your standard of reference.

Thanks for the clarification.