1.
A ~ is a thin layer of something that has formed on the surface of something.
He allowed a fine ~ of old coffee to develop around the inside of the mug.
N-SING: with supp
2.
The ~ on an old object is an attractive soft shine that has developed on its surface, usually because it has been used a lot.
...a mahogany door that is golden brown with the ~ of age.
N-SING: with supp
3.
If you say that someone has a ~ of a quality or characteristic, you mean that they have a small but impressive amount of this quality or characteristic.
...a superficial ~ of knowledge...
N-SING: with supp, oft N of n