[sokoloff]

I think it's a quite literal and clear explanation of the differences between the types of plans.

Which variable is fixed?

In a defined contribution plan, the contributions are fixed/defined and the benefits vary.

In a defined benefit plan, the benefits are defined/fixed and the contributions vary.

[hammock]

That's the spin coming through. In a traditional pension plan, the company pays you a defined amount for your retirement. In a "defined contribution" plan, the employee pays for their own retirement (maybe some of what they pay gets matched).