Definition - What does Wing mean?
When a grapevine develops a cluster of berries or fruit, there are many parts that branch off from the main cluster to form new parts of the plant. The wing is the secondary or smaller arm that forms at the branch at the top of the main cluster.
There are wing-bearing and non-wing bearing clusters in the plant Vitis Vinifera, and the difference between the two is their weight and yield. Clusters with wings are heavier (even after the wings are removed) and produce a greater yield.
WineFrog explains Wing
The vine has several different parts that contribute to the formation of the wings. First, the main axis, also known as the rachis, is made up of the main arms and secondary or lateral wings, also called the outer arms. The inner arm will develop either a wing or a large branch at the top of the main cluster and grows laterally along the main axis (rachis). Both the inner arms and wings give rise to branches and then to more clusters which expand the growth of the vine. When measuring midseason growth, it was found that clusters that developed wings or secondary arms had a higher volume or weight than those clusters that were non-wing bearing.
Wings are separated from the main cluster and are suspended, just like tendrils, with the assistance of a bract. Not only did wing-bearing clusters have a heavier volume, but they also develop a denser, thicker, larger diameter rachis (main axis) which strengthens the vine overall.