What is Jus Sanguinis?

Legal Definition
Jus sanguinis (Latin: right of blood) is a principle of nationality law by which citizenship is not determined by place of birth but by having one or both parents who are citizens of the state. Children at birth may automatically be citizens if their parents have state citizenship or national identities of ethnic, cultural, or other origins. Citizenship can also apply to children whose parents belong to a diaspora and were not themselves citizens of the state conferring citizenship. This principle contrasts with jus soli (Latin: right of soil).
-- Wikipedia
Legal Definition
A person's citizenship is by 'blood', that of the parents. This rule is followed in most countries of the world. Canada and US use Jus Solis (where born) instead.