Flexible Spending Account (FSA)

Pre-tax payroll set-aside through an employer's cafeteria plan for qualified medical expenses — famously 'use it or lose it' (only a small IRS-set amount rolls over). No HDHP required, but a general-purpose FSA makes you HSA/Archer-MSA-ineligible (a limited-purpose FSA — dental/vision only — is the workaround); an FSA can sit alongside an HRA.