You know you're driving through an affluent neighborhood when you see large houses, perfect landscaping, and expensive cars. Use affluent to describe wealthy people or areas.
In Middle English, affluent meant "abundant, flowing," from Old French, from Latin affluēns, from affluere "to abound in," from the prefix ad- "to, at" plus fluere "to flow." The meaning of "abundant, flowing" is still seen in phrases such as "affluent prose." But mostly, when you use the word affluent, the thing understood to be flowing is cash.