Macedonia (/ˌmæsɪˈdoʊniə/ ^ⓘ MASS-ih-DOH-nee-ə) is a geographical and historical region of the Balkan Peninsula in Southeast Europe. Its boundaries have changed considerably over time; however, it came to be defined as the modern geographical region by the mid-19th century. Today the region is considered to include parts of six Balkan countries: all of North Macedonia, large parts of Greece and Bulgaria, and smaller parts of Albania, Serbia, and Kosovo. Greek Macedonia comprises about half of Macedonia's area and population.

Its oldest known settlements date back approximately to 7,000 BC. From the middle of the 4th century BC, the Kingdom of Macedon became the dominant power on the Balkan Peninsula; since then Macedonia has had a diverse history.

Both proper nouns Makedṓn and Makednós are morphologically derived from the Ancient Greek adjective makednós meaning "tall, slim", and are related to the term Macedonia.

The definition of Macedonia has changed several times throughout history. Prior to its expansion under Alexander the Great, the ancient kingdom of Macedonia, to which the modern region owes its name, lay entirely within the central and western parts of the current Greek province of Macedonia and consisted of 17 provinces/districts or eparchies (Ancient Greek: επαρχία).

Expansion of Kingdom of Macedon:

In the 2nd century, Macedonia covered approximately the area where it is considered to be today, but the northern regions of today Republic of North Macedonia were not identified as Macedonian lands. For reasons that are still unclear, over the next eleven centuries Macedonia's location was changed significantly. The Roman province of Macedonia consisted of what is today Northern and Central Greece, much of the geographical area of the Republic of North Macedonia and southeast Albania. Simply put, the Romans created a much larger administrative area under that name than the original ancient Macedon. In late Roman times, the provincial boundaries were reorganized to form the Diocese of Macedonia, consisting of most of modern mainland Greece right across the Aegean to include Crete, southern Albania, southwest Bulgaria, and most of Republic of North Macedonia.

In the Byzantine Empire, a province under the name of Macedonia was carved out of the original Theme of Thrace, which was well east of the Struma River. This thema variously included parts of Thrace and gave its name to the Macedonian dynasty. Hence, Byzantine documents of this era that mention Macedonia are most probably referring to the Macedonian thema. The region of Macedonia, on the other hand, which was ruled by the First Bulgarian Empire throughout the 9th and the 10th century, was incorporated into the Byzantine Empire in 1018 as the Themе of Bulgaria.

With the gradual conquest of southeastern Europe by the Ottomans in the late 14th century, the name of Macedonia disappeared as an administrative designation for several centuries and was rarely displayed on maps. The name was again revived to mean a distinct geographical region in the 19th century, defining the region bounded by Mount Olympus, the Pindus range, mounts Shar and Osogovo, the western Rhodopes, the lower course of the river Mesta (Greek Nestos) and the Aegean Sea, developing roughly the same borders that it has today.