1The number ten thousand (10,000). Used constantly in everyday life as a unit for money, population, distance, and more (e.g., 一万円 '10,000 yen', 一万人 '10,000 people').
この財布には一万円しか入っていない。
There's only ten thousand yen in this wallet.
その町の人口は約三万人だそうだ。
I heard that town's population is about thirty thousand.
東京から大阪まで約五百キロ、飛行機なら一万メートル以上の高さを飛ぶ。
It's about 500 kilometers from Tokyo to Osaka, and a plane flies at over ten thousand meters.
契約書には旧字体の「萬」が使われていることがある。
Contracts sometimes use the old-style character 萬 for this number.
2Meaning 'myriad' — an extremely large or countless number of something. Mostly seen in compounds like 万感 (countless emotions) or 万人 (everyone/all people).
彼の演技を見て、観客は万感の思いに包まれた。
Watching his performance, the audience was overwhelmed with countless emotions.
万人に愛される作品を作るのは簡単ではない。
Creating a work loved by absolutely everyone is not easy.
3Meaning 'everything' or 'all' — used without limiting scope, as in 万事 (all matters) or 万物 (all things).
準備は万事整った。あとは本番を待つだけだ。
Everything is fully prepared. Now we just wait for the real thing.
万物は常に変化し続けている、という考え方が仏教にはある。
Buddhism holds the idea that all things are constantly changing.
あの店では万（よろず）の日用品が揃うと評判だ。
That shop has a reputation for stocking every kind of everyday item.
4(Archaic) Meaning 'various' or 'many different kinds'. Essentially unused in modern Japanese; found mainly in classical texts and poetry.