What Does the Vertical Line in a Partial Differential Equation Indicate?

[Firepanda]

Say if I have a solid vertical line on the right of partial differential equation, with a subscript to that line, i.e:

d |
dx|y

Hard to write out so I hope you can tell what it is. What does the line mean?

 

[Dick]

It generally indicates the variable held constant during the partial differentiation.