I don't usually teach English or Grammar, but I'll give it a try.
First, you have TWO words here and to ask for a definition you need to ask
for it based each word individually.
I'll ignore the definition of word "tax" since that seems to NOT be the
issue. But I'd like to note that in this case, TAX is a qualifier or
modifier, meaning it qualifies or modifies the meaning of the word parlance
to tax issues.
The word "parlance" means "a way or manner of speaking; vernacular; idiom".
For example, Tax Parlance generally MEANS speaking in terms or words that
are specific to the qualifier tax. For example "In bureaucratic parlance,
your proposal would be described as cost effective. In football parlance,
it's a touch down."
Specific to tax situations, parlance would be used to describe how one spoke
about a tax topic - In tax parlance, your deduction does not rise to the
level of a sustainable deduction under IRC 162. In common parlance, you
lose the tax deduction!"