Without further qualification, usually refers to Germanicus Julius Caesar
Claudianus (15 BC - AD 19), father of the Emperor Caligula
and elder brother of Claudius
Born Nero Claudius Drusus in 15BC, his father - also called Nero Claudius Drusus, younger brother of the future emperor Tiberius - died on campaign after a fall from his horse when the boy was only six years old. Both his sons were subsequently awarded the cognomen of Germanicus in memory of their father's successful campaigns against the German tribes, and the elder boy was always subequently referred to by this name in order to distinguish him from Tiberius' son, who also had the same name. His mother was Antonia Minor, the second of Mark Anthony's two daughters by Octavia, sister of the Emperor Augustus.
When Augustus adopted Tiberius as his heir in AD 4, a condition of the adoption was that Tiberius in turn adopt Germanicus - this in spite of the fact that Tiberius' own son was still living, as experience had taught the Emperor that multiple heirs were necessary. As he was now a Julian rather than a Claudian, his official name became Germanicus Julius Caesar Claudianus - an appelation he was to keep fot the rest of his life. That same year, he married Agrippina the Elder, the daughter of Marcus Vipsanius Agrippa and Augustus' daughter Julia. The couple were to have nine children, of whom six would survive their father
Germanicus began his political carreer as quaestor in AD 7, and his military one serving under Tiberius in Germany the following three years - first on the Danube, then on the Rhine. He returned to Rome in 12 to stand as consul, and then replaced his uncle as Governor of Lower Germany in 13.
Augustus died in 14, which made Germanicus second in the line of sucession after Tiberius' own son. He was, by this time, increasingly popular with the ranks of the army - helped, no doubt, by the display of his youngest son Gaius dressed as a miniature soldier. Over the next two years he lead numerous incursions into the German interior; most notably recovering two of the three standards that had been lost six years earlier by Varus. For this last, he was awarded a full triumphal parade by Tiberius - the last time anyone other than a sitting Emperor was granted the honour.
He was consul again in 17, and in 18 was given an imperium maius over the Eastern Mediterranian, which meant he outranked anyone in the area exept the Emperor himself. Once he arrived in his base of Antioch in Syria, however, he immediately clashed with Cn. Calpurnius Piso, who had been appointed Governor of the province by Tiberius at the same time. Piso's family had traditionally been Republicans, and it was rumoured that Tiberius had appointed the Governor with the specific intention of keeping his nephew in check.
In AD19 Germanicus toured Egypt, and on his return found that Piso had reorganised the province in a way contrary to arrangements that he himself had made. He responded by banishing Piso from his company, and the latter retired to an island off the coast, ready to return at a moment's notice should anything happen.
Something did. On October 10, AD19, Germanicus died suddenly. Rumours of poison soon spread - his skin was covered in blue marks, his corpse foamed at the mouth, his heart would not burn on the funeral pyre. The focus of all these rumours was Piso, who was believed to have killed Germanicus on the Emperor's orders because he believed that his nephew was becoming a rival. This last was certainly true amongst the lower classes, who looked on Germanicus as the hero who had salvaged Roman honour from the Germans, and when Agrippina arrived in Rome bearing her husband's ashes, the grief of the nation went with her.
Piso was tried in the Senate for treason and murder, but committed suicide before the verdict could be passed. A recently discovered tablet bearing a record of the trial suggests that perhaps the rumours were indeed true - Tiberius seems to have been on the defensive throughout the proceedings, and even dead, Germanicus was still a potential rival. Caligula's claim to the imperial throne rested entirely on him and both Claudius and Nero (whose mother, Agrippina the Younger, was Germanicus' daughter), made great play of their relationship to him in their dealings with the populace.
Sources: Suetonius' Twelve Caesars and I, Claudius by Robert Graves