Inflation
The rise in prices over time, which means each dollar buys a little less each year.
In the US, inflation is most often measured by the Consumer Price Index from the Bureau of Labor Statistics. It tracks the cost of a set basket of goods and services.
Inflation is why cash sitting in a 0% account slowly loses buying power. Savings that earn more than inflation hold their value. Long-term goals, like retirement, need growth that beats inflation.
Example
At 3% inflation, groceries that cost $100 this year cost about $100 × 1.03 = $103 next year.