乃至 / 甚至于: and even, up to
连词 / conjunctions
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
乃至 (written) marks the extreme end of a scale: "and even", "up to", "not to mention". It often links growing scopes: 个人、国家乃至全世界 (the individual, the nation, even the whole world).
甚至于 is a slightly more colloquial 甚至; 乃至于 also exists. Versus 甚至: 乃至 is mostly used between noun phrases and in formal register.
Examples
这场金融危机影响了整个亚洲，乃至全世界。
Zhè chǎng jīnróng wēijī yǐngxiǎng le zhěnggè Yàzhōu, nǎizhì quán shìjiè.This financial crisis affected all of Asia, and indeed the whole world.
一个小小的发明，可能改变一个行业乃至整个社会。
Yí ge xiǎoxiǎo de fāmíng, kěnéng gǎibiàn yí ge hángyè nǎizhì zhěnggè shèhuì.A tiny invention can change an industry, even society as a whole.
他的作品在国内乃至国际上都很有影响。
Tā de zuòpǐn zài guónèi nǎizhì guójì shang dōu hěn yǒu yǐngxiǎng.His work is influential at home and even internationally.
Common mistakes
✗ 乃至 towards something smaller: ✗ 全世界乃至中国 → the order must rise: ✓ 中国乃至全世界.
✗ Linking everyday verbs: ✓ 甚至 is more natural.