WordNet sense 1 ((music) without breaks between notes; smooth and connected):

legato

WordNet sense 2 (smooth and soft to sight or hearing or touch or taste):

velvet, velvety

WordNet sense 3 (designed or arranged to offer the least resistant to fluid flow):

flowing, sleek

WordNet sense 4 (having a smooth, gleaming surface):

glossy, satiny, silken, silky, slick

WordNet sense 5 (free from obstructions):

smooth out

WordNet sense 6 (make smooth or smoother, as if by rubbing):

smoothen

WordNet sense 7 (smooth and unconstrained in movement):

fluent, fluid, liquid

WordNet sense 8 (make (a surface) shine):

polish, shine, polish

WordNet sense 9 (smoothly agreeable and courteous with a degree of sophistication):

bland, politic, suave

WordNet sense 10 (skilled in dealing with sensitive matters or people):

diplomatical

WordNet sense 11 (posing no difficulty; requiring little effort):

easy

WordNet sense 12 ((of a body of water) free from disturbance by heavy waves):

placid, quiet, still, tranquil, unruffled

WordNet sense 13 ((of weather) free from storm or wind):

calm