space between two Jobes of a leaf (contrast with lobe).
Merriam-Webster Online Dictionary
sinus (noun) as - cavity hollow
a) a narrow elongated tract extending from a focus of suppuration and serving for the discharge of pus
b) (1) a cavity in the substance of a bone of the skull that usually communicates with the nostrils and contains air
(2) a channel for venous blood
(3) a dilatation in a bodily canal or vessel
c) a cleft or indentation between adjoining lobes (as of a leaf or corolla)