smoothsmoothsmoothssmoothedsmoothingsmoothersmoothestadjective (smoother, smoothest)1having a completely flat surface: Babies have such smooth skin. The surface should be completely smooth.oppositerough2with no big pieces in it: Beat the sauce until it is smooth.oppositelumpy3A smooth movement or journey is even and comfortable: The weather was good so we had a very smooth flight.oppositebumpy▶ smoothlyadverb: The plane landed smoothly.