In MV* patters, what exactly is the model?

In MV* patterns, what exactly is the Model which stands for "M"? Is it a POCO? or is it a class that contains your all bulky and reusable business logic?

I read a lot on web (and SO) but the terms are really confusing. Some say model should be bulky which points to the entire reusable business logic. Some say it is your data model which points to the POCO which is replica of your table and its fields.

If it is POCO then how it could be bulky and where should your business logic go?

I have implemented MVC in couple of the projects where my Model is NOT a POCO; it contains reusable business logic. I hope I done it right.