A breakout is the point in a wiring harness where one or more wires leave the main bundle, called the trunk, and run off as a separate branch to a connector or component. It is also called a branch point.
On the harness drawing each breakout has a defined position, measured along the trunk, and each branch has a defined length. The position matters because the branch is tied down or sleeved at that point, and the wires there carry the bending load when the harness is fitted or moves in service.
A breakout is normally secured with tape, a sleeve, heat-shrink or a moulded boot so that the wires cannot slide or rub at the exit. Placing a breakout too close to a connector leaves no room for the wires to bend without stressing the terminals. APAR's harness protection materials chart compares the sleeves, tapes and conduits used at these points.