Nilpotent variety of groups

Definition
A nilpotent variety of groups is a subvariety of the variety of groups (i.e., a collection of groups closed under taking subgroups, quotients, and direct products) satisfying the following equivalent conditions:


 * 1) Every group in it is nilpotent
 * 2) There exists a nonnegative integer $$c$$ such that every group in the collection is nilpotent with nilpotency class at most $$c$$