What does "Sleek" mean?

A friend today asked me what sleek meant and I fumbled an answer ... I knew but it was sort of hard to explain. So I did what every modern man does once in a while ... I googled its definition.

There are a number of explanations but this is the one I liked the most: 

streamlined: designed or arranged to offer the least resistant to fluid flow; "a streamlined convertible"

Ciao :-)

Andrea