Emperor
The first Austrian Emperor was the last Holy Roman Emperor Francis II. In the face of aggressions by Napoleon, Francis feared for the future of the Holy Roman Empire. He wished to maintain his and his family's Imperial status in the event that the Holy Roman Empire should be dissolved, as it indeed was in 1806 when an Austrian-led army suffered a humiliating defeat at the Battle of Austerlitz. After which, the victorious Napoleon proceeded to dismantle the old "Reich" by severing a good portion from the empire and turning it into a separate Confederation of the Rhine. With the size of his imperial realm significantly reduced, Francis II, "Holy Roman Emperor" became Francis I, "Emperor of Austria". The new imperial title may have sounded less prestigious than the old one, but Francis' dynasty continued to rule from Austria and a Habsburg monarch was still an emperor ("Kaiser"), and not just merely a king ("König"), in name.
Holy Roman Empire
A prospective Emperor had first to be elected King of the Romans (Latin: "Rex Romanorum"; German: "römischer König"). German kings had been elected since the 9th century; at that point they were chosen by the leaders of the five most important tribes (the Salian Franks of Lorraine, Ripuarian Franks of Franconia, Saxons, Bavarians, and Swabians). In the Holy Roman Empire, the main dukes and bishops of the kingdom elected the King of the Romans. In 1356, Emperor Charles IV issued the Golden Bull, which limited the electors to seven: the King of Bohemia, the Count Palatine of the Rhine, the Duke of Saxony, the Margrave of Brandenburg, and the archbishops of Cologne, Mainz, and Trier. During the Thirty Years' War, the Duke of Bavaria was given the right to vote as the eighth elector, and the Duke of Brunswick-Lüneburg (colloquially, Hanover) was granted a ninth electorate; additionally, the Napoleonic Wars resulted in several electorates being reallocated, but these new electors never voted before the Empire's dissolution. A candidate for election would be expected to offer concessions of land or money to the electors in order to secure their vote.