Not pertinent to the matter under consideration:
— An issue extraneous to the debate.

synonyms: immaterial, impertinent, orthogonal.

Roget 6: derived from without; objective; extrinsic, extrinsical^†; extraneous etc. (foreign) 57; modal, adventitious; ascititious^†, adscititious^†; incidental, accidental, nonessential; contingent, fortuitous.    implanted, ingrafted^†; inculcated, infused.    outward, apparent etc. (external) 220.    ... show more

Roget 10: irrelative^†, irrespective, unrelated; arbitrary; independent, unallied; unconnected, disconnected; adrift, isolated, insular; extraneous, strange, alien, foreign, outlandish, exotic.    not comparable, incommensurable, heterogeneous; unconformable etc. 83.    irrelevant, inapplicable; not pertinent, not to the, purpose; impertinent, inapposite, beside the mark, a propos de bottes [Fr.]; aside from the purpose, away from the purpose, foreign to the purpose, beside the purpose, beside the question, beside the transaction, beside the point; misplaced etc. (intrusive) 24; traveling out of the record.    remote, far-fetched, out of the way, forced, neither here nor there, quite another thing; detached, segregate; disquiparant^†.    multifarious; discordant etc. 24.    incidental, parenthetical, obiter dicta, episodic.    ... show more

Not essential.

Not belonging to that in which it is contained; introduced from an outside source:
— Water free of extraneous matter.

synonym: foreign.

Roget 57: extraneous, foreign, alien, ulterior; tramontane, ultramontane.    excluded etc. 55; inadmissible; exceptional.   

Coming from the outside:
— Extraneous light in the camera spoiled the photograph.
— Relying upon an extraneous income.

synonyms: external, outside.