A half-sour pickle is a kosher dill pickle that was not left in brine long enough to fully ferment into a full-sour pickle. Unlike a full-sour pickle, a half-sour one is still green and crisp. In certain parts of the United States, half-sour pickles are referred to as new dills, and full-sour pickles are referred to as old dills.
The process of soaking cucumbers in brine and salt turns them into sour pickles. This process relies on the bacteria that covers the outside of the cucumbers as they grow. Because the bacteria is routinely removed when cucumbers are commercially harvested and packed, it will only work on freshly harvested ones unless the bacteria is artificially replaced.