what defines a 4.5 generation aircraft?

I always hear this term being thrown around, but what exactly defines a 4.5 generation aircraft? Not in terms of raw specs, but the type of systems it uses that would give it a label of 4.5 instead of 4th generation. Or is it simply a designation used on aircraft developed/produced after a certain time period.

For example, the Rafale, Typhoon and Su-3X are often classified as 4.5, but what about say, the F-16E, F-15 and F-18E/F which are currently using AESA radars, or say the FC-1 and Tejas which are a step behind?