Also called: low-income years, gap years
The low-tax years between the end of paychecks and the start of Social Security and required minimum distributions. Taxable income is unusually low and largely under your control.
These years are the usual window for Roth conversions, Tax-Aware Drawdown and 0% capital gains harvesting, because income can be added at lower rates than will apply later.
If you are buying ACA Marketplace coverage in the same years, added income also reduces the premium tax credit, so the two goals have to be balanced.