Editor asks for π to be written in roman My paper is accepted at MNRAS and in the stage of editing.
There is a section where I wrote an equation using the Greek letter π as to indicate the mathematical constant π.
The editor wrote:

Author: When used to mean the numerical value 3.14, the Greek letter pi must always be written in roman. Please check all the notations throughout the text carefully

I think this request is canned, so I doubt there is any typo or similar here.
What  exactly is π in roman? Shouldn't it be written in Greek? Is it a mistake of the editor or a misunderstanding from my side?
They are referring to the upright version of pi. In the picture below, you can find on the left an italics version and on the right an upright one.

If you're using LaTeX, you can obtain the upright pi with the upgreek package and the command \uppi, see this Q&amp;A on TeX SE.
More details on this convention can be found in the IUPAC Green book, which follows the ISO/IEC 80000 standard. From p. 7 of the linked document:

The overall rule is that symbols representing physical quantities or variables are italic, but symbols representing units, mathematical constants, or labels, are roman.

And from p. 8:

The symbols π (3.141 592. . .), e (2.718 281. . . , base of natural logarithms), i (square root of minus one), etc. are always roman [...]