Aquincum (Latin: [aˈkᶣɪŋkũː], Hungarian: [ˈɒkviŋkum]) was an ancient city, situated on the northeastern borders of the province of Pannonia within the Roman Empire.

Aquincum served as a military base (castrum), having been part of the Roman border protection system called limes.

[4] Being the centre of operations on the Roman frontier against the neighbouring Iazyges, Aquincum was occasionally the headquarters of emperors.

[2] The city had at least 30,000 inhabitants by the end of the 2nd century, and covered a significant part of the area today known as the Óbuda district within Budapest.

[5] The decline of the Roman Empire also affected Aquincum, and the ancient city was largely destroyed by 350 AD.