What is this 10^40 thing?


While checking out "正" (as in 正坐, specifically) I found that apparently one of
the associated meaning would be 10^40 (i.e. 10 elevated at the 40th power).

Example -[taken from
Tangorin.com](http://tangorin.com/dict.php?dict=general&s=%E6%AD%A3):

sei 【正】 noun / noun with genitive case particle の:

  1. (logical) true; regular 
  2. 10^40 
  3. original —Abbreviation. → せいほん【正本】
  4. positive; greater than zero; —Mathematics term. → ふ【負】

10^40 is a pretty large number (estimated number of water molecules on Earth:
approx. 4.5x10^48) and I wonder if this is to be intended as "infinite", or if
not, when is this specific "meaning" supposed to be used.




It is legit. From [大辞林](http://kotobank.jp/word/%E6%AD%A3?dic=daijirin)

> ⑦ 数の単位。澗（かん）の1万倍，すなわち10の40乗。 〔塵劫記〕

Also, from the [Wikipedia JP page on "正
(数)"](http://ja.wikipedia.org/wiki/%E6%AD%A3_\(%E6%95%B0\)).

> 正（せい）は漢字文化圏における数の単位の一つ。正がいくつを示すかは時代や地域により異なるが、現在では1040を示す。

You can see a list of these numerical terms for large numbers on [the
Wikipedia JP page on
"命数法"](http://ja.wikipedia.org/wiki/%E5%91%BD%E6%95%B0%E6%B3%95#.E5.A4.A7.E6.95.B0).
Oh, [here's an English
version](http://en.wikipedia.org/wiki/Japanese_numerals#Large_numbers).

That said, while 大辞林 has the corresponding entry for each in the list, 大辞泉 and
広辞苑 do not list the definition for everything from (1024) to 極 (1048)
inclusive.

Additionally, the [Wikipedia JP page on "京
(数)"](http://ja.wikipedia.org/wiki/%E4%BA%AC_\(%E6%95%B0\)) mentions that

> ただし、いずれの国でも京まで使われることは稀であり、台湾・韓国では京以上の命数はあまり知られていない。

i.e. the terms for 1016 (京) and upwards are rarely used nowadays.




It is legit. From [大辞林](http://kotobank.jp/word/%E6%AD%A3?dic=daijirin)

> ⑦ 数の単位。澗（かん）の1万倍，すなわち10の40乗。 〔塵劫記〕

Also, from the [Wikipedia JP page on "正
(数)"](http://ja.wikipedia.org/wiki/%E6%AD%A3_\(%E6%95%B0\)).

> 正（せい）は漢字文化圏における数の単位の一つ。正がいくつを示すかは時代や地域により異なるが、現在では1040を示す。

You can see a list of these numerical terms for large numbers on [the
Wikipedia JP page on
"命数法"](http://ja.wikipedia.org/wiki/%E5%91%BD%E6%95%B0%E6%B3%95#.E5.A4.A7.E6.95.B0).
Oh, [here's an English
version](http://en.wikipedia.org/wiki/Japanese_numerals#Large_numbers).

That said, while 大辞林 has the corresponding entry for each in the list, 大辞泉 and
広辞苑 do not list the definition for everything from (1024) to 極 (1048)
inclusive.

Additionally, the [Wikipedia JP page on "京
(数)"](http://ja.wikipedia.org/wiki/%E4%BA%AC_\(%E6%95%B0\)) mentions that

> ただし、いずれの国でも京まで使われることは稀であり、台湾・韓国では京以上の命数はあまり知られていない。

i.e. the terms for 1016 (京) and upwards are rarely used nowadays.

