1. The name of One of the Greek Furies.
2. A Greek city-state, located between Athens and Sparta. Historically, Megara started out under the control of Athens during the Bronze Age. It held a couple of important colonies (Chalcedon and Byzantium) as well as the island of Salamis which was later taken over by Athens in 570 BC. It was also at Megara that the Persian armies were stopped from their military conquests into the Peloponnesian isthmus, and was constantly fought over during the Peloponnesian War by the Athenians and the Spartans, as it is strategically located at the land crossing to the Peloponnese.
When Megara turned against the Delian League, Pericles passed the Megarian Degree which forbade Megarians from Attic markets and harbors (Attica being the general name for the southern peninsula of Greece containing Athens, Piraeus, Marathon, &c). Many historians think this action helped start the Peloponnesian War.
During the war, the Athenians invaded Megara's land holdings and often blockaded the city from the sea, but never actually broke through the main city walls. For this reason Megara is one of the few states that was independent during the war.
The land itself is not very arable -- in fact just about the only natural resource the state has is its supply of salt from the shores. Isocrates said that the Megarians "farmed rocks." Megara was also the birthplace of Theognis the poet.
3. Also one of Hercules' wives. In the Disney version, she's his one true love, but in the original Greek myth she's just one of his many wives and mistresses, and ultimately kills the hero. In the Spanish-language version of the Disney film, the voice of Megara was played by Tatiana. (Hercules was done by Ricky Martin.)
Only the ruins of Megara remain today after the city was destroyed by fire in a battle where Sparta and Athens vied for possession.