The word myriad [ˈmɪɹi.æd/ˈmɪɹi.əd] means a countless number or multitude, and in the past it meant 10,000. It comes from the French myriade (myriad, 10,000), from the Latin Latin myrias (10,000), from the Ancient Greek μυριάς (muriás – countless, 10,000), from μῡρῐ́ος (mūríos – numberless, countless, infinite) [source].
A myriad of stars
The use of 10,000 to mean countless or infinite happens in other languages as well. For example in Chinese 万 [萬] (wàn) means 10,000 or a great number [source]. The same character 万 (man) in Japanese means 10,000, a myriad, everything, all or various. When pronounced ban it means completely, absolutely or totally [source].
Do other languages do something similar?
Other English words that refer to a large but unspecified number include um(p)teen or umpty, which come from umpty (a colloquial name for a dash in Morse Code used as World War I army slang) and -teen [source].
Also zillion, gazillion, bazillion, jillion, bajillion and squillion [source].
Do you have any others?
In Sweden one may say for example “du har frågat för femtielfte gången”,
meaning “you have asked for the umpteenth time”, that is for too many a time.
I think this “femtielfte” really is an odd word. It behaves exactly like the other ordinal numbers (especially the -teen words, 13th to 19th), but is obviously not an ordinary ordinal number.
Literally it means 50th + 11th, which with a proper ordinal would be the sixty-first,
and represents an unspecified large ordinal number.
I love using percent (%), permille (‰), and the best of all, a permyriad (‱).
How about ‘googol’ (ten to the power of 100, i.e., one followed by one hundred zeroes. This word was the origin, in 1998, of the name ‘Google’. There is also ‘googolplex’ (one to the power of one googol zeroes).
Sorry – that last parenthetical phrase should have read “(ten to the power of one googol)”.
I say “on bin” at Turkish (Southwestern Turkic) and “on miñ” at Tatar (Northwestern Turkic) languages for 10 000. But, Ancient Turkic language don’t use “on biñ” for 10 000, Ancient Turkic language used “Bir Tümen” (One Myriad) word.
In Modern Turkish, we use the “tümen” word forever. But, it means only “division” (military unit) unfortunately. The numeral sense is obsolete..
Ancient Greek: μυριάς (myriás)
East Asian: 万 [萬] (wàn, man, etc.)
Ancient Turkic: 𐱅𐰇𐰢𐰤 (tümen)
1 0000 = 10 000 = 10⁴
100 Myriad means One Million.
100 0000 = 1 000 000 = 10⁶
Turkic: 𐰘𐰇𐰔:𐱅𐰇𐰢𐰤 (Yüz Tümen)
East Asian: 百萬 / 百万 (Hyaku Man, Baek Man, etc.)
Greek: εκατομμύριο (ekatommýrio)