What does decoherence have to do with phases?

[Talisman]

Here is Artur Ekert, a major figure in QIS, giving the single-qubit + environment example a year ago:

The textbook version is here https://qubit.guide/12.2-decoherence-and-interference.html

For better or worse, this usage does indeed seem to well-established in this particular field.

 

[kith]

As others have noted, "decoherence" is used differently in different communities. I've found the section "A Few Words on Nomenclature" in Klaus Hornberger's 2009 lecture notes Introduction to decoherence helpful in the past. It gave me an overview of the different usages of "decoherece" and how it relates to similar concepts like "dephasing".