Smooth

douce

(linguistics, classical studies, of a vowel: lacking marked aspiration)

doux

(without difficulty)

facile

(without difficulty)

lisse

(lacking friction, not rough)

lisse

(bland; glib)

lisse

(mathematics, of a function: Having derivatives of all finite orders at all points within the function’s domain)

naturel

(action: natural; unconstrained)

régulier

(mathematics, of a function: Having derivatives of all finite orders at all points within the function’s domain)

sophistiqué

(suave; sophisticated)

souple

(motion: unbroken)

lisser

(make smooth)

lisser

(statistics: capture important patterns in the data, while leaving out noise)