A somewhat antiquated acidulant that has gained a renaissance of popularity towards the end of the twentieth century. It's origins are a little cloudy (somewhat like the product itself), but it has definitely been produced since medieval times. Unlike vinegar, which has grape juice or wine acidulated by an introduced culture, verjuice is simply crushed, unripe fruit juice. Grapes are the normal ingredient, but I have heard of apples and crab apples being used as well.
It is much less acidic than vinegar or lemon juice and has a pleasing sweetness that adds a lovely mellow taste to savoury dishes and desserts alike. It can be added to a vinaigrette, or to deglaze a saute pan that has been used to cook poultry. It is an essential ingredient in the ancient Agresto sauce, indeed, agresto is Italian for unripe grape juice.
They only commercial producer of verjuice that I am aware of is Australian chef, Maggie Beer. But if you grow your own grapes, I imagine it would not be too difficult to make. Just crush them before they are fully ripe.
Also known as Verjus