Diophantine Geometry: An Introduction
《Diophantine Geometry: An Introduction》 · 第 131 页 · Section A.7.3「Dual Abelian Varieties and Poincaré Divisors」，定义段
A polarization is said to be a principal polarization if $K(c)=\{O\}$. Thus $c$ gives a principal polarization if the map $\Phi_c:A\to\widehat A$ is an isomorphism.