[ghaff]

When people say "pension" at least in the US they usually mean a defined benefit retirement plan, i.e. you get $X/month based on salary and years of service starting at age Y.

By contrast, a 401(K) is a defined contribution plan. You put however much money in, maybe get matching, and you can take money out after a certain age based on how much you put in and how much it grew.

Defined benefit used to be a lot more common but it's become less so for a variety of reasons including tax law changes. Today you see them mostly in government jobs like teachers.