You have a couple of choices, but don't think of it as a variable, think of it as state. When you start thinking of it as state, you can start asking questions around storage, manipulation and/or access.
You could always use session state. You can access this from any controller or view. By default, this is stored in memory and lost when a user's session dies and/or your application pool is reset on the server. It is tied to a single user.
Session["YourVariable"] = "Customer Name";
or
Session["Customers"] = new List<customer>();</customer>
or similar.
Better would be to store this information in a database. There are many resources for this here on the site and across the web.
Cheers.
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