fluctus

Fluctus, fluctus is fourth declension masculine, from fluo, "to flow". Like every fourth declension noun it hides its plural: fluctus is nominative singular, nominative plural and accusative plural all at once, and only the dative and ablative plural fluctibus is unmistakable.

Against unda, which is water in motion and the ordinary word for a wave, fluctus is the bigger, rougher thing, a breaker or a swell, and it is the poet's choice. The figurative use follows naturally: fluctus rei publicae, the storms of politics, and the fluctus animi of a mind in turmoil. English fluctuate comes from the derived verb fluctuo, and the same fluo root gives fluid, influence, affluent and flux.