I have looked through the docs, and have not seen any reference to a #define type of declaration. I used var as a surrogate. This has several problems. First of all, the var becomes part of the object in the project and any subsequent attempt to reassign the value in the declaration fails. I solved this by changing to a private var, which makes the value local to the object and not saved in the project.
This leaves the issue of a defined value that is used across objects which as far as I can tell requires a var. Which returns us to the problem of changing the assigned value for this kind of declaration in MonoDevelop.
I am still in week 1 as a Unity developer, so pardon my deep ignorance, and thanks in advance for your helpful response. BTW, Unity is impressive in its ability in getting a project up and running quickly.