She was so nervous to see her high school sweetheart at their reunion: it had been a decade (ten years) since she'd seen him at graduation, and she still got butterflies thinking about him.
A decade is a period of ten years. Decade contains dec, which is used in words that have to do with tens. The Marty Paich Dectet was a band with ten players. A decahedron is a shape with ten faces. And the decimal system is based on the number ten.