Hilbert's Nullstellensatz (German for "theorem of zeros," or more literally, "zero-locus-theorem" – see Satz) is a theorem which establishes a fundamental relationship between geometry and algebra. This relationship is the basis of algebraic geometry, an important branch of mathematics. It relates algebraic sets to ideals in polynomial rings over algebraically closed fields. This relationship was discovered by David Hilbert who proved Nullstellensatz and several other important related theorems named after him (like Hilbert's basis theorem).
Read more about Hilbert's Nullstellensatz: Formulation, Proof and Generalization, Effective Nullstellensatz, Projective Nullstellensatz