円滑【えんかつ】
smooth, undisturbed, uninterrupted, harmonious
滑らか【なめらか】
smooth (of a surface), glassy, velvety, soft, smooth (of an action, proceedings, etc.), fluent (speech), fluid, trouble-free, continuously differentiable
潤滑油【じゅんかつゆ】
lubricating oil, lubricant, lube, person or thing that makes things run smoothly, facilitator
滑る【すべる】
to glide, to slide (e.g. on skis), to slip, to fail (an examination), to bomb (when telling a joke), to drop, to go down, to come down, to fall (e.g. in status)
滑稽【こっけい】
funny, comical, humorous, laughable, ridiculous, silly, absurd
滑り台【すべりだい】
(playground) slide, (ship) launching platform, sliding bed