What .Net Namespace Contains Entity For Use In A Generic Repository

I have a question that I'm ashamed to ask, but I'm going to have a go at it anyway. I am creating a generic repository in asp.net mvc. I came across an example on this website which I find to be exactly what I was looking for, but there is one problem. It references an object - Entity - and I don't know what namespace it is in. I typically create my repositories and use Entity Framework but I decided to use a generic repository because I am using the same code in multiple projects over and over again.

Here is the code:

public interface IRepository
{
void Save(ENTITY entity)
where ENTITY : Entity;
void Delete<ENTITY>(ENTITY entity)
where ENTITY : Entity;

[code]....

what namespace Entity is in? As you can tell, a constraint is placed on the code so that it must be an Entity type. I know that there is an Entity in System.Data.Entity, but that isn't what I need. I have had instances before where I was looking for some namespace that took me forever to find, but I have searched and I'm unable to find the appropriate namespace to cast my generic items correctly. I could cast it as a class and be done with it, but it is bugging me that I can't find Entity anywhere.