a mina: a fixed weight of silver used as money, worth about a hundred days' wages for an ordinary worker
A mina was not a coin, and no mint ever struck one. It was a set weight of silver, counted as a hundred drachmas (δραχμή), the Greek silver coin that paid a working man for about a day. So one mina came to roughly a hundred days of wages, close to three months of earnings for an ordinary laborer.
In the New Testament the word belongs to one passage only: the parable Jesus tells in Luke 19 as he nears Jerusalem. A nobleman leaving to be made king calls ten servants, hands each of them a single mina, and tells them to trade with it until he comes back. On his return one servant has made ten more, a second five, and a third has kept his wrapped in a cloth and earned nothing.
The size of the sum carries part of the story. A mina was real money, plenty to do business with, yet the master treats it as a very small thing beside what he then hands out, authority over whole cities. On the ancient scale it sits well below the top unit, the talent (τάλαντον), which was worth sixty minas, and well above a single day's pay, the denarius (δηνάριον).
The same unit runs through the Old Testament as the Hebrew mina (מָנֶה), a weight of silver or gold in temple and treasury accounts, so this was an old and familiar way of measuring money long before Luke wrote.