Sideline vs Undefined - What's the difference?

As a noun sideline

is a line at the side of something, as in "the yellow sideline of the road".

is to place on the sidelines; to bench or to keep someone out of play.

lacking a definition or value.

A line at the side of something, as in "the yellow sideline of the road".

(sports) A line defining the side boundary of a playing field.

(usually, in the plural) The area outside the playing field beyond each sideline.

The coach stood on the sidelines and bellowed commands at the team.

The outside or perimeter of any activity.

She installed the whole fixture while he simply watched from the sidelines .

Something that is additional or extra or that exists around the edges or margins of a main item.

She started the business as a sideline to her regular work and it ended up becoming the greater source of income.

Soup need not be just a sideline to a meal; if you like, it can be the main course.

(sidelin)

To place on the sidelines; to bench or to keep someone out of play.

The coach sidelined the player until he regained his strength.

To remove or keep out of circulation.

The illness sidelined him for weeks.

(wikipedia undefined) (-)

Lacking a definition or value.

(mathematics, computing) That does not have a meaning and is thus not assigned an interpretation.

The result of division by zero is undefined .

* defined