It says in Wikipedia:
Corollary: The eigenvalues of $A$ must also lie within the Gershgorin discs $C_j$ corresponding to the columns of $A$.
Before that, a Gershgorin disc is defined as.
$D(a_{ii}, R_i)$ be the closed disc centered at aii with radius $R_i$. Such a disc is called a Gershgorin disc.
A Gershgorin disc is defined for a row, so what does "the Gershgorin discs $C_j$ corresponding to the columns of $A$" mean?
Thanks in advance!