Sideline
A sideline is an alternative continuation outside the branch treated as the main line.
A sideline is not automatically weak. It may be less common, less studied, or chosen to reach a different kind of position.
Compare a central route with a sideline
After 1.e4 e5 2.Nf3 Nc6 3.Bb5, 3...a6 enters the heavily studied Morphy Defense. The immediate 3...Nd4 is Bird's Defense, an established alternative usually treated as a sideline.
Starting position · Position 0/6
A sideline can be a practical repertoire choice
A sound sideline can be a practical repertoire choice when it reduces memorization or steers the game toward a familiar structure. The tradeoff is that uncommon moves can also lead to positions with less published guidance.
Prepare sidelines that appear in your games
A branch can be rare in master databases but common at your rating or time control. Personal game frequency is therefore a better training signal than the sideline label alone.