This term is used to describe crop loss due to impaired pollination. Normally this will mean missing grapes from within the cluster rather than the loss of the whole cluster. The most common cause of shatter is heavy rain, hail or extreme heat.
Another condition occurring in early grape development. The tiny grapes have a small "cap" on the end. Normally, as they start to grow, this cap pops off. However, in a cool, wet spring, such as in 1995, the grape fails to develop early and the cap toughens. When warmer weather finally hits and the grape grows, it pushes against this tough cap. Instead of the cap popping off, the grape shatters. Again, this drastically reduces yields and leads to very low berry count and bunches with only a few grapes. Shatter was a major factor in the very low yields of the 1995 harvest.
To break at once into many pieces; to dash, burst, or part violently into fragments; to rend into splinters; as, an explosion shatters a rock or a bomb; too much steam shatters a boiler; an oak is shattered by lightning.