Definition - What does Cluster mean?
A cluster refers to the bunches of grape berries that grow on a vine. Its formation begins in the dormant bud where the cluster primordia is found. This is the anatomy of a vine which produces fruit. It is called "dormant" as it does not grow during the same season as when it forms itself.
WineFrog explains Cluster
The cluster is the part of the vine from which fruit is grown. The dormant bud where the cluster develops is formed during the vine's dormant period. It then remains dormant within the vine until the following growing season. This is when leaves, tendrils and then flower clusters sprout from the vine.
It consists of flower clusters which will then be pollinated and grow the fruit (grape) clusters. The amount of fruit found on a cluster often varies according to each specific grape variety and the growing conditions in which the grapes are planted.