Jus soli (latin for "right of the territory") is a right by which nationality or citizenship can be recognised to any individual born in the territory of the related state. It contrasts with jus sanguinis ("right of blood")
Usually a practical regulation of the acquisition of nationality or citizenship of a state by birth on the territory of the state is provided by a derivative law called lex soli. Most states provide a specific lex soli, in application of the respective jus soli, and it is the most common means of acquiring nationality.
A frequent exception to lex soli is opposed when a child was born to a parent in the diplomatic or consular service of another state, on a mission to the state in question.
However, increasingly countries are restricting lex soli by requiring that at least one of the child's parents be a national of the state in question at the child's birth, or a legal permanent resident of the territory of the state in question at the child's birth, or that the child be a foundling found on the territory of the state in question.
Sometimes, a country which extends jus soli will ignore it in the case of the child of a parent who is later deported. For example, there are several cases of deportation from Canada which have been bitterly contested because they involve a mother who has given birth in Canada and whose child is therefore a Canadian citizen. The government must then either separate the mother and her child upon deporting her, or else deport a Canadian citizen to a foreign country.
A minor, quite improper use of the term jus soli refers to the jurisdiction: in this case it would indicate that the law to use is the law of the nation/state in which territory the evaluated fact happened. But, as said, it is not considered a correct use of the term, or at least is it considered misleading.