What does reduce by half mean?

  What does it mean when someone wants you to reduce the balsamic by half?  Cook books just expect you to know, right?  Well, I had to google this one.  Basically, reducing is a term that shows up in a lot of recipes, and apparently you're just supposed to know what it means — whether you've cooked much or not. Really, it just means to evaporate a portion of the liquid;  wine, stock, sauce, etc. Often you'll see a direction to reduce a sauce by some fraction, perhaps a half, so using your best judgment, you evaporate and evaporate until only half the original volume is left.  Reducing a liquid makes it thicker and intensifies the flavor.