Liquidity
How quickly you can access your money without incurring fees, penalties, or losses. Cash in a savings account is highly liquid; cash in a 5-year CD is much less so.
Liquidity is a measure of how quickly an asset can be converted to cash you can spend, without losing value. A high-yield savings account is one of the most liquid asset types available — you can typically transfer your money to a checking account within 1–3 business days at no cost.
Compare this with: a CD (locked up for the term, with an early withdrawal penalty if you break it), a brokerage account holding stocks (settles in 1–2 days but values fluctuate), real estate (weeks to months to sell), or a 401(k) before retirement (10% penalty plus taxes if withdrawn early).
For an emergency fund or any short-horizon savings goal, liquidity matters more than yield. The best HYSA in the world isn't worth much if you cannot get to the money when you need it.