Donativum (plural donativa) was the name given to the gifts of money dispersed to the soldiers of the Roman legions or to the Praetorian Guard by the Roman Emperors. The English translation is donative.
The purpose of the donativa varied: some were expressions of gratitude for favors received, and others outright bribery for favors expected in return. Donativa were normally rendered at the beginning of each new emperor's reign. During the 2nd and 3rd centuries, this form of bribery became a crucial part of any successful ruler in Rome. Such was the case with many of the soldier-emperors from 235 to 248.
The Praetorian Guard, intimate to the emperor's person, was an even greater threat to security. The cohorts stationed in Rome were difficult to appease and quick to commit assassination. The donativum thus provided a significant way to purchase the Guards' support and loyalty.
Emperor Augustus bequeathed the Praetorian Guard a substantial sum in his will, but it was not until the reign of Tiberius that gifts of money were thought mandatory. The Praetorian Guard received such gifts for turning a blind eye when Sejanus, their prefect, fell from power. Each Praetorian Guard received 10 gold pieces for refraining from defending Sejanus's.
In 41, after the assassination of Caligula, the Guard supported Claudius, and after a brief time the Senate learned that the Guard had installed him on the throne. Claudius gave them 150 gold pieces, or some 3,750 denarii, to which the senators' 100 sesterces were added annually to commemorate Claudius's accession. The inevitable result of the custom of the donativum was the Praetorians' auctioning of the Empire to Didius Julianus in 193
IMPERIAL DONATIVA TO THE PRAETORIAN GUARD 14-193 AD
YEAR EMPEROR PROVOCATION DENARII
14 Augustus Last will 250
31 Tiberius Loyalty in Sejanus crisis 1,000
37 Caligula Upon accession 500
41 Claudius Upon accession
Annually Claudius Anniversary of accession to the throne 3,750
54 Nero Accession 3,750
Pay for assassinations 500 or less
Promised by Nymphidius Sabinus, but not paid 7,500
69 Galba Promised 1,250
69 Otho Promised 1,250
69 Vitellius Regular donativum unknown
69 Vespasian Regular donativum unknown
79 Titus Regular donativum unknown
81 Domitian Considered doubling the donativum but opted for regular sum unknown
98 Nerva Regular donativum unknown
117 Trajan Regular donativum unknown
138 Hadrian Double normal sum unknown
161 Antonius Pius Regular donativum and upon daughter’s marriage unknown
180 Marcus Aurelius Joint rule with Verus 5,000
193 Commodus Regular donativum; second promised but unpaid unknown
193 Pertinax Force to pay donativum of Commodus 3,000
193 Didius Julianus Purchased the throne 7,250 but paid less
193 Septimius Severus Promised a donativum 250
https://en.wikipedia.org/wiki/Donativum