A pension is a plan where your employer promises to pay you money _after you retire_, in perpetuity. In the "olden days", or when working for the government, one often would work at the same employer for 20+ years, and a pension is a way for that employer to support you. (My dad has multiple pensions from when he worked at a large company, and then later for the local county government.) In general, it scales up the longer you worked for a company. This was back when companies and employees were "loyal" to one another.

That seems to be a lot less common now.

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