Nullspace

The Subspace defined by the Set of $\vec{x}$ that satisfy $A\vec{x}=\vec{0}$

$$\text{Nul}A=\{\vec{x}|A\vec{x}=\vec{0}\}$$ $$\dim \text{Nul}A=\text{\# free vars}=\text{Nullity}A$$

See Nullity