noun
depending on; according to; nature of
Formal noun used after another noun (+ の) to mean 'depending on ...' or 'the nature of ...'. Often appears in set phrases like いかんによって or いかんにかかわらず.
結果は努力いかんだ。
The result depends on your effort.
天候のいかんによっては、試合が中止になることもある。
Depending on the weather, the match may be cancelled.
adverb
what is ...?; how is ...?; what will become of ...?
Formal sentence-ending adverb used to ask about the state or outcome of something, often with a nuance of concern or uncertainty. Common in written or rhetorical questions.
この計画の行方はいかん。
What will become of this plan?
彼の真意やいかん。
What are his true intentions?
Rarely used kanji form; learners should prefer 如何.
どう is the common casual word for 'how' or 'what'. いかん is much more formal and often used in set expressions or written language.
いかに is an adverb meaning 'how' or 'in what way', often used in formal or literary contexts. いかん is a noun or sentence-ending adverb with a more specific 'depending on' or 'what will become of' nuance.
Derived from classical Japanese. The kanji 如何 is a jukujikun (special reading) for いかん, not reflecting the individual character meanings. The exact historical derivation is uncertain.