also: なんか
pronoun
Indefinite pronoun referring to an unspecified thing. Used in questions, negative sentences, and conditional clauses.
何か食べるものはありますか。
Is there something to eat?
何かあったら連絡してください。
Please contact me if anything happens.
adverb
Adverbial use expressing that something is felt or perceived in an unclear way. Often used with ～となく or ～知らず.
何か変だと思った。
I thought something was off.
何か知らないけど、うまくいった。
I don't know why, but it went well.
interjection
what (do you mean)?; so what?
Only with reading なにか
Interjection used to question someone's vague or evasive statement. Restricted to the reading なにか.
「あのさ…」「何か？」
"Hey..." "What?"
何も is used in negative sentences to mean 'nothing' or 'not at all', while 何か is used in affirmative or interrogative contexts.
何でも means 'anything' or 'whatever', implying no restriction, whereas 何か refers to an unspecified but limited thing.
Compound of interrogative 何 (なに) and question particle か. The contracted form なんか is a common colloquial variant.