The expression round pound is a simple compositional creation, using round as an adjective in the sense defined in the Macmillan Dictionary Online as ‘… given as a whole number or a number ending in zero.’ Its road to popular use also benefits from what linguists refer to as assonance (repetition of vowel sounds). The term often occurs as a modifier to another noun, and is sometimes hyphenated in this position, e.g. round-pound sales, round-pound pricing, etc.