"Legato - Smooth Piano Playing"

Legato is an Italian word meaning "smooth" or "tied together" -- which is how you're meant to play legato passages on the piano. Legato passages are often written with the notes under a curved line called a slur or phrase mark. Sometimes composers simply write the word "legato" in the score.

Legatto, leggato

His legato playing in the upper register of the piano was a joy to hear; such smooth, silky sounds must surely mark him out as a supreme master of articulation.