Someone who sings opera professionally.

(derogatory) a person with mediocre or amateur singing capabilities.

The main vocalist in a musical group / A singer (a machine that singes) made out of lead (the element)

(differential geometry) A theorem stating that, for an elliptic differential operator on a compact manifold, the analytical index (related to the dimension of the space of solutions) is equal to the topological index (defined in terms of some topological data).