Why are judicial circuits named "judicial circuits"?
The answer by Dale M is basically correct but has some details wrong.
US Courts of Appeal
Recall that in the US Federal system the circuit courts are the "Courts of Appeal", that is the middle level of the system, above the district courts, but below the Supreme Court.
In the early days of the US, each Justice of the Supreme Court was assigned to one circuit, usually consisting of several adjacent states. The Justice spent roughly half of each year "riding circuit". The Justice would stop at two to four places in the circuit, where Federal district courts were located.The justice would sit together with one of the district court judges in those places, in the courthouse that would be otherwise used for the district court. That Two-judge panel would constitute the session of the Circuit Court. The places where the circuit court was held were usually state capitols or other large towns or cities. The Justice would not stop at every small town along the way.
(I recently read The Marshall Court and Cultural Change: 1815-1835, Which describes the practice in detail. Travel was quite wearing on the Justices. During the actual Supreme Court term, they lived in a boardinghouse in Washington DC, usually the same house for all the Justices. Cases were often discussed after hours at these boardinghouses. The Chief Justice often made arrangements for the coming year's accommodation. The Justices' wives did not normally join them in Washington nor on circuit.)
This practice persisted until 1912, but had ceased by the early twentieth century. It stopped in some Circuits sooner than others. There are now permanent judges assigned to the Courts of Appeal, which is now their proper formal name, but the areas over which they have jurisdiction are still known as circuits, and the courts are informally still called the Circuit courts.
In fact the judges of the Courts of appeal are often where Presidents look when there is a vacancy on the Supreme Court.
Each circuit still has a supervising Justice assigned to it. That Justice handles certain "emergency" appeals from the circuit court, either ruling on them at once, or relaying them to the full Supreme Court.
English Assize Circuits
In England, before the US existed, judges rode circuit. These assize circuits were defined by statute in 1293, and remained in use until they were replaced by the Crown Court in 1972 under the Courts Act 1971
In the West and Mid-west of the US during the nineteenth century, some state courts (not federal courts) rode circuit in areas where the population was not dense enough to support a full time stationary court. The Judge was often accompanied by a group of lawyers, whom would take on the various cases that awaited the judge. However, these courts are not what is usually meant by "circuit courts" in US usage. These judges did often stop at a relatively large number of small towns, often county seats or comparable locations, where a county courthouse might exist.
This practice stopped at different times in different areas, but it was obsolete by the early twentieth century.