This is a term to describe a main, usually central, stem (usually referring to woody perennials).

This often refers to trees, more specifically the training of standard trees, where a ‘central leader’ stem is trained to grow vertically upwards to ‘lead’ the tree’s growth. In some cases the central leader is removed to encourage lateral growth and, therefore, a less columnar or conical shape.

The longest growing shoot from lateral branches (the one that appears to be responsible for extending the length of the branch) is known as the ‘branch leader’.