1780, coined by Jeremy Bentham from inter- "between" + national (adj.). In the phrase international jurisprudence. He footnotes the word with:
The word international, it must be acknowledged, is a new one; though, it is hoped, sufficiently analogous and intelligible. It is calculated to express, in a more significant way, the branch of law which goes commonly under the name of the law of nations: an appellation so uncharacteristic, that, were it not for the force of custom, it would seem rather to refer to internal jurisprudence. [Bentham, "Introduction to the Principles of Morals and Legislation"]
As a noun and with a capital -i-, it is short for International Working Men's Association, a socialistic worker's movement with global aims, the first chapter of which was founded in London by Marx in 1864. The group lends its name to "The Internationale" (from fem. of French international, which is from English), the socialist hymn, written 1871 by Eugène Pottier. International Dateline is from 1882. Related: Internationally (1821).