Short answers, pulled from the story.
The Hellenistic period began with the death of Alexander the Great on the 10th of June 323 BC and ended when Cleopatra VII committed suicide in 30 BC. Most academic consensus settles on 31/30 BC as the definitive endpoint because it marks the final elimination of the last major Hellenistic kingdom by Roman forces.
A 19th-century German historian named Johann Gustav Droysen coined the phrase to describe how Greek culture spread into non-Greek regions after Alexander's conquests. This label distinguishes the broad cultural influence across Asia, Africa, and Europe from the narrower concept of Hellenic which refers only to Greece proper.
This conflict established the tripartite division that would define Hellenistic politics: Macedon under Antigonus II Gonatas, Egypt under Ptolemaic rulers, and Syria under the Seleucids. The Seleucid Empire expanded to include central Anatolia, the Levant, Mesopotamia, Persia, Turkmenistan, Pamir, and parts of Pakistan at its height.
No complete historical works survive from the first hundred years following Alexander's death so historians rely on fragmented accounts from writers like Hieronymus of Cardia, Duris of Samos, and Phylarchus whose original texts are now lost. Polybius of Megalopolis provides the earliest credible surviving source writing his Histories between 220 and 167 BC while serving as a hostage in Rome.
Greek colonists migrated eastward to establish new cities across Asia Minor, Egypt, and Central Asia bringing their language which evolved into Koine Greek an Attic-based dialect that became the common tongue throughout the ancient world. Indigenous populations adopted Greek customs deemed beneficial or necessary for governance and commerce including Thrace nobility adopting Greek fashions in dress ornament and military equipment.