天下

Earliest attestations are in the Book of Documents and Classic of Poetry (as「天之下」then「天下」).

Pines (2002) thinks it "likely that "originally tianxia referred to the area under the direct rule of the Son of Heaven [a.k.a. the Zhou king], and its limits might have shrunk together with the contraction of royal power". See there for more on the term's gradual semantic evolution through the Spring and Autumn period, Warring States period, until after Qin's wars of unification.

天下

1. (literary, figurative) the world; everything under the sky; all under heaven

   • 天下莫大於秋豪之末，而大山為小；莫壽於殤子，而彭祖為夭。天地與我並存，而萬物與我為一。 [Classical Chinese, trad.]
     天下莫大于秋豪之末，而大山为小；莫寿于殇子，而彭祖为夭。天地与我并存，而万物与我为一。 [Classical Chinese, simp.]

     From: c. 369 B.C.E. – 286 B.C.E., 莊子 (Zhuangzi), 齊物論 (The Identity of Contraries)

     Tiānxià mò dàyú qiūháo zhī mò, ér dàshān wéi xiǎo; mò shòuyú shāngzǐ, ér Péngzǔ wéi yāo. Tiāndì yǔ wǒ bìngcún, ér wànwù yǔ wǒ wèi yī. [Pinyin]

     There is nothing under the canopy of heaven greater than the tip of an autumn spikelet. A vast mountain is a small thing. Neither is there any age greater than that of a child cut off in infancy. P'êng Tsu himself died young. The universe and I came into being together; and I, and everything therein, are One.

   • 奴才替天下百姓感激皇太后恩典！ [MSC, trad. and simp.]

     From: The Firmament of the Pleiades

     Núcái tì tiānxià bǎixìng gǎnjī huángtàihòu ēndiǎn! [Pinyin]

     I, on behalf of the people throughout the world, [hereby] express my gratitude for the kindness of the empress dowager!

2. (literary, lofty) China; the Chinese nation

   統一天下／统一天下  ―  tǒngyī tiānxià  ―  to unify China (literally, “to unify all under heaven”)

   • 及至始皇，奮六世之餘烈，振長策而御宇內，吞二周而亡諸侯，履至尊而制六合，執棰拊以鞭笞天下，威振四海。 [Classical Chinese, trad.]
     及至始皇，奋六世之余烈，振长策而御宇内，吞二周而亡诸侯，履至尊而制六合，执棰拊以鞭笞天下，威振四海。 [Classical Chinese, simp.]

     From: The Records of the Grand Historian, by Sima Qian, c. 91 BCE

     Jí zhì Shǐhuáng, fèn liù shì zhī yúliè, zhèn chángcè ér yù yǔnèi, tūn èr zhōu ér wáng zhūhóu, lǚ zhìzūn ér zhì liùhé, zhí chuífǔ yǐ biānchī tiānxià, wēizhèn sìhǎi. [Pinyin]

     (please add an English translation of this usage example)

   • 今天下三分，益州疲弊，此誠危急存亡之秋也。 [Classical Chinese, trad.]
     今天下三分，益州疲弊，此诚危急存亡之秋也。 [Classical Chinese, simp.]

     From: 227, 諸葛亮 (Zhuge Liang), 出師表

     Jīn tiānxià sānfēn, Yìzhōu píbì, cǐ chéng wēijí cúnwáng zhī qiū yě. [Pinyin]

     Today, China is divided into three parts, and the Kingdom of Han is weak and fatigued. This is truly a critical moment determining our survival.

3. (literary, figurative) state power; domination

• (the world):

• (China):

Others:

• → Manchu: ᠠᠪᡴᠠᡳ
  ᡶᡝᠵᡝᡵᡤᡳ (abkai fejergi) (calque)
• → Russian: Поднебе́сная (Podnebésnaja) (calque)

From Middle Chinese 天下 (MC then haeX, “heaven + under”). The kan'on, so likely a later borrowing.[1]

• (Tokyo) て​んか [téꜜǹkà] (Atamadaka – [1])[2][3][4]
• IPA^(key): [tẽ̞ŋka̠]

天(てん)下(か) • (tenka) 

1. the whole world, the earth
2. the whole country, all of Japan
3. the government of Japan
4. power, control over the government
5. (historical, Edo period) the shogun
6. (card games) in traditional Japanese card games, one of the strong cards known as 蠣 (aza)
7. short for 天下一 (tenka ichi): the best in the world (often used attributively with the particle の (no))

   天(てん)下(か)の横(よこ)綱(づな)

   tenka no yokozuna

   the best yokozuna in the world

From Middle Chinese 天下 (MC then haeX, “heaven + under”). The go'on, so likely the earlier borrowing.[1]

Appears with this reading in the Kagerō Nikki, dating to roughly 974 CE.[2] Superseded in modern Japanese by tenka above.

• IPA^(key): [tẽ̞ŋɡe̞]

天(てん)下(げ) • (tenge) 

1. (archaic, possibly obsolete) the world, the earth

• 天(てん)上(じょう) (tenjō)
• 地(ち)上(じょう) (chijō)
• 地(ち)下(か) (chika)
• 天(あま)下(くだ)る (amakudaru)

天下 • (cheonha) (hangeul 천하)

1. Hanja form^? of 천하 (“world, earth; China”).

天下

1. chữ Hán form of thiên hạ (“world”).