Definition of "meniscus"

And from Crabbe's own forehead sweat dripped or gathered into a kind of meniscus to be scooped off.

1959, Anthony Burgess, Beds in the East (The Malayan Trilogy), published 1972, page 554

He opened wide both casements; they gave on a parking place four floors below; the thin meniscus overhead was too wan to illumine the roofs of the houses descending toward the invisible lake [...].

1972, Vladimir Nabokov, Transparent Things, McGraw-Hill, published 1972, page 19