Repeater vs Undefined - What's the difference?

As a noun repeater

is (electronics) an electronic device that receives a weak or low-level signal and retransmits it at a higher level or higher power.

lacking a definition or value.

(en noun)

(electronics) An electronic device that receives a weak or low-level signal and retransmits it at a higher level or higher power.

(firearms) A gun that has a store of cartridges and does not need reloading after each shot.

In ufology and similar studies, a person who regularly sees unexplained sightings of paranormal phenomena.

Note – this term is more commonly used by skeptics of the paranormal, and implies that the witness lacks credibility.

A watch with a striking apparatus which, upon pressure of a spring, will indicate the time, usually in hours and quarters.

(US) One who votes more than once at an election.

A repeating decimal.

(nautical) A pennant used to indicate that a certain flag in a hoist of signal is duplicated.

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(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