Image Circle

Hi, can some optical expert very kindly explain, or refer me to a source, what the definition of Image Circle is and what is the significance of that value. How does this relate to eveness of illumination for astrophotography? For example, if scope A has an image circle of 44mm and scope B has an image circle of 88mm, what does that imply? If Scope A has an image circle of 44mm, is this a gaurantee that the illumination is very even within that circle?

Thanks very much,
Ivan